Xen: finishing the job

The PV driver backends for all VirtIO devices are present in the mainline QEMU codebase - the VirtIO backends don't care about the type of virtualization - all they want is ability to provide emulated PCI devices. So QEMU, KQEMU, KVM all work just fine in this regard, though obviously the best performance will be when you combine them with KVM. You could also probably make the VirtIO backends work under Xen fullvirt too..