For all intents and purposes, the functionality of Xen HVM and KVM are almost identical. This is because they rely on hardware virtualization SVM or Intel VT, and they are based on the QEMU Hypervisor as are most hardware assisted Virtualization Technologies.
Performance wise we've seen nearly identical results and examples from both technologies which is no surprise. The only difference is from drivers optimized for either platform in our experience.
Xen is certainly more mature and has a richer feature set than KVM. Xen has also proven itself to be more stable than KVM in many situations and for this reason we recommend and prefer Xen, but still offer KVM servers for those who want them.