The folks behind the Xen Orchestra project have run some interesting benchmarks comparing a classic Xen PV configuration against Xen PVHVM (Hardware Assisted Virtualization with specialized drivers) on Debian Linux. With Debian 7, PVHVM is supported "out of the box" by its kernel.
The Xen Orchestra crew found PVHVM Debian Xen guests to be faster than Xen's classic para-virtualization and "you can get more performance for your current Xen infrastructure for a small cost." Those interested in seeing the full benchmarks can find the details on the Xen Orchestra blog. Their testing was driven in part by the Phoronix Test Suite so you can also find more of their Xen performance data on OpenBenchmarking.org. Their tests were in turn inspired by Rackspace benchmarks, which are also wanting a full run of the Phoronix Test Suite to show off to their customers.