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.
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.