QEMU

QEMU emulates a complete machine in software, so an operating system or binary built for one CPU architecture runs on a host with a different one. With no hardware virtualization it emulates everything; paired with KVM or Xen it hands the CPU to the hypervisor and runs guests at near-native speed. This is the engine underneath much of the Linux virtualization stack.

It also does userspace emulation for Linux and BSD, running a binary built for one ABI on a host with another. The command line and monitor API are stable, so QEMU is driven directly or through higher-level layers such as libvirt.

QEMU is GPL-2 licensed and builds on Linux, macOS, and Windows via Mingw64, plus other UNIX targets. Source lives in Git, releases ship as tarballs, and changes are submitted as email patches rather than pull requests.