I never really realized that Firecracker VM is a full-blown machine and not just some sort of a Linux container tech. At first, it may sound like an ineffective approach, but if you take a closer look on a real-world usage example such as fly.io, you will be surprised: micro-VMs are very small and capable.

Thanks to KVM, and to the minimal hardware support (no PCI, no ACPI, etc), Firecracker's source is rather simple and even relatively readable for non-experts.