If you have the knowledge and the ability to hand edit the configuration file on Xen PV, you can actually get better performance over KVM. But if you use the stock configuration files like Virtualizor, Solus, and Promox generate, KVM will give better performance.
We have spent many hours experimenting with both configurations and actually use both in our offerings. Dedicated resources we use Xen HVPV, shared resources we use KVM. Its something that you need you play with and run tests for yourself.