What is PVOPS?
PVOPS is a piece of Linux kernel infrastructure to allow Linux to run paravirtualized on a hypervisor. PVOPS is not technically a Xen Sub-project, but a crucial dependency for the Hypervisor and thus worth listing here. PVOPS currently supports VMWare's VMI, Rusty's lguest, and most importantly, the Xen Project binaries.
The PVOPS infrastructure allows you to compile a single kernel binary which will either boot native on bare hardware (or in HVM mode under Xen), or boot fully paravirtualized in any of the environments you have enabled in the kernel configuration. This includes the Dom0 of the Xen Hypervisor (which is the first guest that the Xen Hypervisor boots).
The current status of Xen PVOPS can be found in the Xen Project Wiki here.