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I think you're talking about two different technologies, here. The CPU support for Virtualization Technology (VT) should be turned on and left on. There's absolutely no reason to turn it off - it must be on for you to run an HVM domU, anyway. I also don't know of any adverse effects of turning on VT (or AMD's matching technology). HVM = Hardware Virtual Machine, which requires VT and which completely emulates a full hardware set for the guest. Paravirtualization requires that a guest kernel be modified to support PV technology. When a PV kernel and/or drivers are present and installed correctly, they will always be used and always should be used. In the absence of PV support in a guest, HVM must be used. A guest can either run a PV kernel or a standard kernel. If it runs a PV kernel, it is a PV domU, if it runs a standard kernel, it must be run inside an HVM domU. If you're running an O/S that has a PV kernel available (Linux, BSD, Solaris), you should probably use a PV domU, although there may be certain situations where this is not desirable.
This gets even a little more confusing because HVM domUs can have PV drivers that bypass some of the hardware emulation to provide improved performance (e.g. the Windows GPL PV drivers by James Harper). If you have access to these drivers, you should probably use them - the performance improvement is generally pretty good, and there's not usually a reason to use the standard ones.
-Nick
>>> On 2009/06/09 at 03:25, "Nikola K. Zahariev" <nirangor@xxxxxxxxx> wrote:
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Hello!
I have a newbie question regarding the hardware virtualization in XEN.
When is actually hardware virtualization being used?
If we assume the CPU supports HV, does XEN use it by default?
Is there a possibility to switch it off, so that paravirtualization is used?
How can I check if I am currently using HV or PV?
Thanks in advance!
Best regards,
Nikola
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