Xen users - May 2011 - HVM vs PV (Xen Full virtualization (HVM) Xen Paravirtualization (PV))

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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Hi Nemus!

 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
<YOUR EMULATED PCI DEVICES LISTED HERE>


* 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.

Att,
Thiago

On 5 May 2011 16:40, <nemus@grayhatlabs.com> 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/
>
>
> _______________________________________________
> Xen-users mailing list
> Xen-users@lists.xensource.com
> http://lists.xensource.com/xen-users
>

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> 
> Windows does not work as a PVM domain.
> 
That's correct, but it is possible (and desirable) to load drivers into
Windows that use the PV communications channelto talk to the disk/network/etc in
Dom0 instead of the emulated qemu PCI devices. This is true under HVM Linux too.
HVM also has some other benefits too.

(you would know this obviously, but I just thought I'd add it for the
OP's benefit :)

James

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You right!!! I forgot this details... Maybe that is because I never used
Windows LOL
So, I never needed the HVM domains...

Thiago

2011/5/18 James Harper <james.harper@bendigoit.com.au>
> >
> > Windows does not work as a PVM domain.
> >
>
> That''s correct, but it is possible (and desirable) to load drivers
into
> Windows that use the PV communications channelto talk to the
> disk/network/etc in Dom0 instead of the emulated qemu PCI devices. This is
> true under HVM Linux too. HVM also has some other benefits too.
>
> (you would know this obviously, but I just thought I''d add it for
the OP''s
> benefit :)
>
> James
>

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Thank you for the reply.

So the debate I have been having is. Even with the new cpu technology that
helps out the virtualization PV is still a better option right?

So if you had a choice between hvm and pv you should using pv because it
will have an over all better performance.

Is that correct?

Thank you.
> Hi Nemus!
>
>  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
> <YOUR EMULATED PCI DEVICES LISTED HERE>
>
>
> * 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.
>
> Att,
> Thiago
>
> On 5 May 2011 16:40, <nemus@grayhatlabs.com> 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/
>>
>>
>> _______________________________________________
>> Xen-users mailing list
>> Xen-users@lists.xensource.com
>> http://lists.xensource.com/xen-users
>>
> _______________________________________________
> Xen-users mailing list
> Xen-users@lists.xensource.com
> http://lists.xensource.com/xen-users



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