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by peterwwillis 4123 days ago
tl;dr, Windows. But also various limitations of how PV or HVM hosts work.

PVH fixes it so you don't need emulation in Linux, but it's a brand new feature and probably not production quality. Read this (it's not too technical) http://wiki.xen.org/wiki/Virtualization_Spectrum

As of this writing, Xen 4.4 and Linux 3.14 have experimental support for PVH DomUs and Xen 4.5 has support for PVH Dom0s. PVH allows practically native-hardware-speed guests without any emulation.
1 comments
cthalupa 4123 days ago
Completely eliminating emulation isn't always desirable, unfortunately. The problem with PVH is the forcing the use of event channels to deliver interrupts.

Local APIC emulation is fully accelerated on modern processors, which is a big win for any use case that is heavy on interrupts.

link