QEMU: the FAST! processor emulator Full-system emulation - Run operating systems for any machine, on any supported architecture User-mode emulation - Run programs for another Linux/BSD target, on any supported architecture Virtualization - Run KVM, Xen Virtual
UTM's backend is a modified version of the well known QEMU, with iOS-specific changes and additions. Among these is an acceleration layer called TCTI for fast emulation while remaining a safe, sandboxed app.