Xen on Linux supports para-virtualization (PV) as a means of virtualization without depending upon any CPU virtualization extensions like AMD SVM or Intel VT-x. PVH, on the other hand, is a means of Xen para-virtualization that leverages the processor's hardware extensions for faster PV support. PVH is a new feature to Xen 4.4.
The Xen Wiki describes the new PVH virtualization mode as, "essentially a PV guest using PV drivers for boot and I/O. Otherwise it uses HW virtualization extensions, without the need for emulation. First patches are going into xen-unstable just after the Xen 4.3 release and functionality should be at least available as a pre-view in Xen 4.4. PVH has the potential to combine the best trade-offs of all virtualization modes, while simplifying the Xen architecture."
The only problem with Xen PVH is that the mainline Linux kernel doesn't support it even though patches were posted long ago, they got lost in the clutter. In Konrad's new mailing list post he noted, "Mukesh had posted them quite a while ago - they got reviewed, acked, etc, put in #linux-next, got patches to fix some things from folks. Then everybody focused on the Xen patches .. and now that is mostly baked this is what is needed on the Linux side to make it work with PVH...Decided to rebase all of them on v3.13-rc3 to make it easier."
Hopefully the Xen PVH support will be merged then for the Linux 3.14 kernel merge window in January. The official release of Xen 4.4 is also expected in January.