Firecracker vs Cloud Hypervisor

Firecracker and Cloud Hypervisor are both open-source Virtual Machine Monitors written in Rust that use KVM to create lightweight VMs for cloud workloads. They share the same underlying philosophy (minimal device models, small attack surfaces, and fast boot times) but differ in scope, feature set, guest OS support, and the use cases they are designed for.

This article compares the two on architecture, performance, features, operational complexity, and when each is the right choice.

Firecracker is an open-source VMM built by Amazon Web Services, released in 2018 under the Apache 2.0 licence. It was purpose-built for multi-tenant serverless and container workloads, specifically to power AWS Lambda and AWS Fargate. It started from crosvm, the Chromium OS VMM written in Rust, and has diverged significantly since.

Firecracker's design philosophy is strict minimalism. It implements only five devices: virtio-net, virtio-block, virtio-vsock, a serial console, and a minimal keyboard controller. There is no graphics, no USB, no BIOS, no ACPI. This minimal device model keeps the attack surface small and boot times fast. Each microVM initiates user-space code in as little as 125ms, with less than 5 MiB of memory overhead per instance. It supports up to 150 microVMs per second per host in benchmarks.

Firecracker runs on 64-bit Intel, AMD, and Arm CPUs with hardware virtualisation support, using KVM as its hypervisor backend. It supports Linux and OSv guests. It includes a built-in rate limiter per microVM for granular control of network and storage resources, and a RESTful API for VM lifecycle management. A companion process called the jailer provides an additional security layer by further isolating the Firecracker process using Linux namespaces and cgroups in case the virtualisation barrier is compromised.

See What is AWS Firecracker? for a full technical breakdown.

Cloud Hypervisor is an open-source VMM written in Rust, governed under the Linux Foundation and based on the rust-vmm crates. It runs on KVM and Microsoft Hypervisor (MSHV) and targets modern cloud workloads with a broader feature set than Firecracker while maintaining a minimal, auditable codebase.

Where Firecracker draws a hard line at five devices, Cloud Hypervisor supports a wider range of paravirtualised virtio devices without legacy hardware emulation. Its capabilities include boot to userspace in less than 100ms, live migration, CPU and memory hotplug, PCI hotplug, GPU passthrough via VFIO, and a REST API for programmatic VM lifecycle management. It supports x86-64 and AArch64 architectures and runs 64-bit Linux and Windows 10/Windows Server 2019 guests.

Cloud Hypervisor is the primary VMM backend for Kata Containers in production deployments. It is supported by organisations including Alibaba, AMD, Ampere, ARM, ByteDance, Intel, Microsoft, SAP, and Tencent Cloud.

For a full technical breakdown of Cloud Hypervisor's architecture and capabilities, see the guide to Cloud Hypervisor.

Both VMMs share the same core approach: Rust implementation, KVM-based hardware virtualisation, virtio paravirtualised devices, and a focus on modern cloud workloads without legacy hardware emulation. They diverge significantly in scope.

Firecracker optimises for the absolute minimum. Five devices, no Windows support, no live migration, no GPU passthrough, no hotplug. Every feature that does not directly serve its target use case (high-density serverless and container workloads) is excluded. That constraint is a feature: fewer code paths mean fewer vulnerabilities and lower overhead.

Cloud Hypervisor optimises for modern cloud workloads broadly defined. It supports live migration, GPU passthrough, Windows guests, hotplug, and a wider virtio device set while still keeping legacy hardware out. It is more capable than Firecracker but has a larger codebase as a result.

Both use a RESTful API for VM management. Both are supported as VMM backends in Kata Containers. Both use KVM as their primary hypervisor. Cloud Hypervisor additionally supports MSHV, extending its reach to environments where KVM is not available but Microsoft Hypervisor is.

Both VMMs target fast boot and low memory overhead, but measuring them head-to-head requires care because the figures come from different benchmarking conditions.

Firecracker's official documentation states user-space initiation in as little as 125ms. Cloud Hypervisor's official documentation states boot to userspace in less than 100ms. These figures reflect each project's own benchmarks under their own conditions and should not be read as a definitive claim that Cloud Hypervisor is faster than Firecracker in all environments. Real-world performance depends on host hardware, kernel configuration, image size, and workload characteristics.

At runtime, both deliver near-native performance for their guest workloads since the guest kernel handles syscalls directly without an interception layer. Firecracker's rate limiter provides built-in per-microVM throttling for network and storage, which Cloud Hypervisor does not have built-in.

For very high workload density, Firecracker's less-than-5-MiB memory overhead per microVM and support for up to 150 microVM creations per second per host in benchmarks make it well-suited to environments where packing thousands of microVMs on a single host matters.

Both VMMs use KVM hardware virtualisation to enforce isolation and are implemented in Rust, which eliminates a class of memory corruption vulnerabilities common in C-based VMMs.

Firecracker adds a companion process called the jailer that provides a second line of defence. The jailer further isolates the Firecracker process using Linux namespaces and cgroups, so that if the virtualisation boundary were compromised, the attacker's access to the host remains limited. Cloud Hypervisor uses seccomp filtering to restrict which host syscalls the VMM process can make.

Both approaches reduce the host attack surface beyond KVM alone. The choice between them is not meaningfully a security differentiator for most use cases, both are production-hardened and used at scale.

• Live migration: Cloud Hypervisor supports live VM migration from one host to another without interruption. Firecracker does not support live migration. For workloads that need to move between hosts without downtime (long-running services, stateful workloads), Cloud Hypervisor is the right choice.
• GPU passthrough: Cloud Hypervisor supports GPU passthrough via VFIO, allowing VMs to access physical GPU hardware directly. Firecracker does not support GPU passthrough. For isolated GPU workloads, Cloud Hypervisor is required.
• Windows guests: Cloud Hypervisor supports Windows 10 and Windows Server 2019 as guest operating systems. Firecracker supports Linux and OSv guests only. For teams running mixed Linux and Windows workloads, Cloud Hypervisor is the only option.
• CPU and memory hotplug: Cloud Hypervisor supports adding CPU and memory to a running VM without rebooting. Firecracker does not. For long-running workloads that need to scale resources dynamically, Cloud Hypervisor has an advantage.
• Rate limiting: Firecracker includes a built-in rate limiter per microVM for granular network and storage throttling configurable via its API. Cloud Hypervisor does not include built-in rate limiting.
• MSHV support: Cloud Hypervisor runs on Microsoft Hypervisor in addition to KVM, extending its reach to environments where KVM is unavailable. Firecracker is KVM-only.

Neither Firecracker nor Cloud Hypervisor integrates with Kubernetes directly. Both require Kata Containers as the orchestration layer that bridges them to the Container Runtime Interface. You configure Kata with your chosen VMM backend via RuntimeClass, and Kata handles VM provisioning, guest kernel boot, and networking transparently from Kubernetes' perspective. See What are Kata Containers? for how that integration works.

Firecracker also integrates with containerd via the firecracker-containerd project, providing an alternative path for container workloads outside Kubernetes.

Use Firecracker when:

• You are building a custom serverless or FaaS platform and need maximum density and simplicity
• Your workloads run on Linux or OSv with no requirements for GPU passthrough, live migration, or Windows guests
• Built-in per-microVM rate limiting matters for your resource management model
• You want the smallest possible attack surface and codebase
• You have the infrastructure expertise to manage Firecracker directly or are using it through Kata Containers

Use Cloud Hypervisor when:

• You need live migration for long-running or stateful workloads
• Your workloads require GPU passthrough for isolated GPU compute
• You need to run Windows guests alongside Linux workloads
• CPU or memory hotplug is a requirement
• You are running on an environment where MSHV is available but KVM is not
• You want a broader feature set while still maintaining a modern, auditable codebase

For most teams running microVM isolation in Kubernetes via Kata Containers, Cloud Hypervisor is the default and covers the majority of workload requirements. Firecracker is the right choice when density, simplicity, and rate limiting matter more than feature breadth.

Northflank's sandbox infrastructure uses Kata Containers with Cloud Hypervisor as its primary VMM, with Firecracker applied for workloads that benefit from its minimal device model and gVisor applied where syscall-interception isolation is sufficient or where nested virtualisation is unavailable. The platform has been in production since 2021 across startups, public companies, and government deployments.

Sandboxes spin up in approximately 1 to 2 seconds, with compute pricing starting at $0.01667 per vCPU per hour and $0.00833 per GB of memory per hour. See the pricing page for full details.

Northflank supports both ephemeral and persistent sandbox environments on managed cloud or inside your own VPC, self-serve into AWS, GCP, Azure, Oracle, CoreWeave, Civo, on-premises, or bare-metal via bring your own cloud.

Both VMMs target sub-200ms boot times to userspace under their own benchmarking conditions. Firecracker documents user-space initiation in as little as 125ms. Cloud Hypervisor documents boot to userspace in less than 100ms. These figures come from different benchmarks and should not be read as an absolute ranking. Real-world performance depends on host hardware, kernel size, and workload configuration.

No. Firecracker supports Linux and OSv guests. Cloud Hypervisor supports 64-bit Linux and Windows 10/Windows Server 2019 guests.

Yes. Both Firecracker and Cloud Hypervisor are supported VMM backends for Kata Containers. Cloud Hypervisor is the default backend in most Kata Containers production deployments. Firecracker is available as an alternative backend.

No. Firecracker does not support live VM migration. Cloud Hypervisor supports live migration from one host to another without interruption.

The jailer is a companion process that provides a second line of defence for Firecracker microVMs. It further isolates the Firecracker process using Linux namespaces and cgroups, limiting what an attacker can access if the virtualisation barrier were compromised.

Firecracker's device model is more constrained, five devices versus Cloud Hypervisor's broader virtio device set, which means a smaller codebase and fewer potential attack vectors in the VMM itself. Both are implemented in Rust and are production-hardened. For most use cases, the difference is not the deciding factor.

• What is AWS Firecracker?: a full technical breakdown of Firecracker's architecture, device model, and jailer
• What are Kata Containers?: how Kata Containers orchestrates Firecracker and Cloud Hypervisor for Kubernetes workloads
• What is a microVM?: how microVMs work and how both VMMs implement them
• What is KVM?: the hardware virtualisation layer both VMMs build on
• Kata Containers vs Firecracker vs gVisor: a three-way comparison of the leading isolation technologies
• Cloud Hypervisor vs gVisor: how Cloud Hypervisor compares to syscall-interception isolation
• Firecracker vs gVisor: how Firecracker compares to syscall-interception isolation
• Firecracker vs Docker: how microVM isolation compares to standard container isolation