Difference between Xen PV, Xen KVM and HVM?

Xen supported virtualization types

Xen supports running two different types of guests. Xen guests are often called as domUs (unprivileged domains). Both guest types (PV, HVM) can be used at the same time on a single Xen system.

Xen Paravirtualization (PV)

Paravirtualization is an efficient and lightweight virtualization technique introduced by Xen, later adopted also by other virtualization solutions. Paravirtualization doesn't require virtualization extensions from the host CPU. However paravirtualized guests require special kernel that is ported to run natively on Xen, so the guests are aware of the hypervisor and can run efficiently without emulation or virtual emulated hardware. Xen PV guest kernels exist for Linux, NetBSD, FreeBSD, OpenSolaris and Novell Netware operating systems.

PV guests don't have any kind of virtual emulated hardware, but graphical console is still possible using guest pvfb (paravirtual framebuffer). PV guest graphical console can be viewed using VNC client, or Redhat's virt-viewer. There's a separate VNC server in dom0 for each guest's PVFB.

Upstream kernel.org Linux kernels since Linux 2.6.24 include Xen PV guest (domU) support based on the Linux pvops framework, so every upstream Linux kernel can be automatically used as Xen PV guest kernel without any additional patches or modifications.

See XenParavirtOps wiki page for more information about Linux pvops Xen support.

Xen Full virtualization (HVM)

Fully virtualized aka HVM (Hardware Virtual Machine) guests require CPU virtualization extensions from the host CPU (Intel VT, AMD-V). Xen uses modified version of Qemu to emulate full PC hardware, including BIOS, IDE disk controller, VGA graphic adapter, USB controller, network adapter etc for HVM guests. CPU virtualization extensions are used to boost performance of the emulation. Fully virtualized guests don't require special kernel, so for example Windows operating systems can be used as Xen HVM guest. Fully virtualized guests are usually slower than paravirtualized guests, because of the required emulation.

To boost performance fully virtualized HVM guests can use special paravirtual device drivers to bypass the emulation for disk and network IO. Xen Windows HVM guests can use the opensource GPLPV drivers. See XenLinuxPVonHVMdrivers wiki page for more information about Xen PV-on-HVM drivers for Linux HVM guests.

KVM is not Xen at all, it is another technology, where KVM is a Linux native kernel module and not an additional kernel, like Xen. Which makes KVM a better design. the downside here is that KVM is newer than Xen, so it might be lacking some of the features

-------------------------------------------------------------------------------------------------------------------------------

Read following  for more clearance

Full virtualization is a process where an entire computer system is made into a software construct. This construct acts like the original hardware in every way. Software that is designed for the hardware will install on the construct as though it was the actual computer and then run with little to no slowdown. Using full virtualization has several uses, such as testing software in a virtual environment or expanding the usefulness of a single computer or server through virtual operating systems. While partial virtualization is very common, full virtualization is relatively rare.
In order to be full virtualization, an entire hardware system needs to be transformed into software. Every action and nuance of the original hardware needs to move over to the virtual system. Since this is such a large undertaking, and some system manufacturers take steps to discourage it, full virtualization is somewhat rare. It is much more common to find partial virtualization, where all the necessary system bits are present, but the physical hardware system handles much of the low-level calculations and functions.