Gnome Boxes is just another front end to KVM. The minimalist front end to KVM is virsh (CLI).

VirtualBox works on hardware where the ISA does not support virtual machines (or where said, *cough* Intel, instructions are buggy and insecure). Instead of full emulation, it patches the ram of supported OSes to make system calls.

Xen also works on hardware with no ISA support for VMs. (It also supports ISA VM support.) There is a xen micro-kernel that does nothing but run VMs. Even IO is done by a privileged VM. The kernel in the VMs must have Xen device drivers - which today are merged with virtio device drivers (so the same drivers work for both Xen and KVM).