XEN is open sourced vertualization platform and management system, it's also the only available opensource type-1 program for the moment. Administrators be able to run multiple operation systems concurrently, thus it's used in lots of commercial or open source projects such as: Server virtualization, Infrastructure as a Service, Desktop virtualization, Security applications, Embedded and hardware appliances etc. Xen is powering the biggest cloud computing market.
Xen support two kinds of virtualization: PV & HVM, both virtualization can be running on same XEN server. Moreover, PV can be used within HVM virtual server which is popularly named "PV on HVM". What would be the difference between PV and HVM for XEN virtualization?
Xen Paravirtualization (PV)
High efficient and light weight virtualization technology by XEN. Physical CPU doesn't need virtual extension to support PV. However, PV requires Xen-PV-enabled kernel and PV driver in order to power a high performance virtual server. The good thing is most linux destributions already support this including NetBSD, FreeBSD, OpenSolaris etc since from linux kernel version 2.6.24.
Xen Full Virtualization (HVM)
It's also called hardware assist virtualization. HVM makes use of CPU virtual extension from physical machine. HVM requires Intel VT or AMD-V hardware extension. Xen uses Qemu for Simulation of PC hardware including BIOS, IDE, VGA, Network card and USB etc. HVM doesn't need any kernel support, it means windows OS can be used on XENHUM. Because of simulation technologies, HVM performance is not so good as PV machine.
PV on HVM
To improve performance, HVM can also use some special PV drivers (PVHVM or PV-on-HVM). These PV drivers optimize HVM environment and do balance to hard disk & network IO. In this way you get similar or better virtual server than PV. Because of this, you can optimize operation system that only support full virtualization technology, such as windows.
Xen paravirtualization uses PV drivers automatically and no need to provide these drivers seperately, only Xen full virtualization requires PVHUM drivers seperately.
PV in an HVM Container (PVH) – New in Xen 4.4
Xen 4.4 provides a new virtualization mode named PVH. It provides a half virtualization choice for bootup and I/O. Comparing to HVM, PVH uses hardware virtualization extension but no need simulation. PVH be able to balance all virtualization advantages and simplify Xen structure.
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