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PV vs HVM guest on XenServer 7.0

Hi, list:

I am in the process of upgrading my hypervisors clusters from XenServer 6.5 to 7.0 for all my ACS instances.

My XS 7.0 clusters are patched up to XS70E050.

During cluster rolling upgrade, VM instances are live migrated several times, eventually all of them running on XS 7.0 hosts.
However, out of a hundred or so instances, a few of rhle 6.x VM got stuck at “Plex86/Bochs VGABios …” screen with at least one vCPU at 100%.
Comparing VM parameters between running and stuck instances, I can see that running instances are PV guests while stuck instances are HVM guests.
RHEL 7.x guest are always in HVM mode and always runs on both XS 6.5 and 7.0 hosts.

Running RHEL6.x guests:

               HVM-boot-policy ( RW):

               HVM-boot-params (MRW):

         HVM-shadow-multiplier ( RW): 1.000

                       PV-args ( RW): graphical utf8

                 PV-bootloader ( RW): pygrub

Stuck RHEL 6.x guests:

               HVM-boot-policy ( RW): BIOS order

               HVM-boot-params (MRW): order: dc

         HVM-shadow-multiplier ( RW): 1.000

                       PV-args ( RW):

                 PV-bootloader ( RW):

Why does CloudStack convert just a few rhel 6.x instances into HVM mode, while leaving most in PV mode?
How would I force them back to PV guests?

Thanks for all the help