devicenull parent
Er, what? QEMU is used very frequently with KVM. There are many companies providing KVM instances, so QEMU is pretty widely used in production. Did you have a more specific question?
Xen also uses QEMU, in pretty much the same manner as KVM.
That's not entirely true. Xen uses QEMU only to execute I/O. It gets I/O requests on a ring buffer and services them. CPUs are handled by the hypervisor.
With KVM, QEMU actually runs the code for the virtual machine; it just does that inside the KVM kernel module.
This means, for example, that only with KVM you can profile each VCPU simply using "perf". Also, features that require code to run in the CPU threads (for example slowing down CPUs so that migration can converge better) will not work with Xen.