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I have talked about this last week in a XenServer lecture.

1- HVM domains are based on a emlulation of the hardware writen entirely in software (a.k.a. QEmu) and it depends on a assistence by hardware to become useful.

2- PVM domains are the new approach of virtualization technique, simple there is no "hardware emulation via software" within a PVM domain. No emulation at all. It is much more fast!! You just need a old 64Bits CPU to virtualize Linux, some BSDs and OpenSolaris using PVM domains.

Try this:

Install two Debians in your XenServer, one HVM and another PVM:

1- Debian HVM (hostname debian01), at XenCenter click "New VM", select "Other Install media" and choose the original Debian ISO CD image from your "NFS ISO storage repository";

2- Debian PVM (hostname debian02), at XenCenter click "New VM", select "Debian Squeeze 6 32 bits", choose the "Install URL" method and type "http://ftp.debian.org/debian/", configure the network within the VM (better with dhcp server on your network) and install it normally.

After that, you will be able to see the big differences from debian01 (HVM) to debian02 (PVM) with one Linux command:

* HVM Domain has a virtual motherboard, plus virtual PCI devices as: IDE/SCSI block devices, hard disc, video board, all emulated by software:

username@debian01~$ lspci

* PVM Domain have no virtual motherboard or any virtual PCI devices (notthing will appear because there is nothing to show):

username@debian02~$ lspci

username@debian02~$

As you'll figure out, emulation tasks are heavy and slow, the emulation is expensive and a slow proccess, but recently Intel and AMD implemented an assistance for hardware emulation within their CPUs in order to make it feasible to emulation

The disk and network within a PVM domain is just some frontends (blockfront and netfront), that talsk directily to the dom0 (blockback and netback), without any emulation.

Also, it is possible to convert a VM from HVM to PVM after the installation, and vice-versa.

Windows does not work as a PVM domain.

On 5 May 2011 16:40, <nemus@xxxxxxxxxxxxxxx> wrote:

Is HVM or PV better for performance and overall stability in xen?

I asked on Linux question but got mixed results.

http://www.linuxquestions.org/questions/linux-virtualization-90/hvm-vs-pv-xen-full-virtualization-hvm-xen-paravirtualization-pv-872427/

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Re: [Xen-users] HVM vs PV (Xen Full virtualization (HVM) Xen Paravirtualization (PV))