Paravirtualization is not an extension of full virtualization. If anything, it's moving in the other direction - instead of virtualizing the parts of the system

Paravirtualization is not an extension of full virtualization. If anything, it's moving in the other direction - instead of virtualizing the parts of the system that are difficult or low performance when virtualized, you instead offer up a software device. In practice, this is generally your IO devices - without SR-IOV support, your IO is quite slow through an emulated device. Paravirtual IO drivers, you have a front end driver in the guest, and a back end driver elsewhere. In Xen, this is a dom0 or stub domain that also includes the actual hardware driver. With Xen the front end and back end are just a shared memory segment using a ring buffer. KVM does things a bit differently, but the core concept is pretty similar. SR-IOV you have a virtual function of the device that is passed through directly to the guest, eliminating the need for the PV IO drivers at all.