More than one month ago I wrote of the Xen PVH support coming back up and now it's part of the Xen pull request for Linux 3.14. Xen PVH is para-virtualization that uses the CPUs's hardware extensions for faster PV support.
The Xen 3.14 pull request by Oracle's Konrad Rzeszutek Wilk explains PVH at length:
In short, PV is a mode where the kernel lets the hypervisor program page-tables, segments, etc. With EPT/NPT capabilities in current processors, the overhead of doing this in an HVM (Hardware Virtual Machine) container is much lower than the hypervisor doing it for us. In short we let a PV guest run without doing page-table, segment, syscall, etc updates through the hypervisor - instead it is all done within the guest container. It is a "hybrid" PV - hence the 'PVH' name - a PV guest within an HVM container.On the user-space side PVH will be supported with the Xen 4.4 release. Additional information on Xen PVH can be found via the Xen.org Wiki.
The major benefits are less code to deal with - for example we only use one function from the the pv_mmu_ops (which has 39 function calls); faster performance for syscall (no context switches into the hypervisor); less traps on various operations; etc.
It is still being baked - the ABI is not yet set in stone. But it is pretty awesome and we are excited about it.
There's also event channel scalability improvements, bug-fixes, and other work as part of the Xen update for Linux 3.14.
Also submitted for the Linux 3.14 kernel merge window were the first KVM updates but there really isn't anything exciting besides bug-fixes for this alternative virtualization hypervisor.