I'm doing this process right now, and its painfully slow.
If you are upgrading a pool, start with the pool master. Disable HA for the pool, and stop or migrate all VMs off to other pool members.
You have to boot the 6.5 installer CD directly, using physical media or some kind of remote-access card like drac / ilo / rsa / cimc card.
Then the installer prompts for "Upgrade or Clean Install" Choose upgrade, and wait. It asks very few questions compared to a full install.
Reboot once the install is completed and the machine will come up
NOTE this may appear broken on first boot. DON'T PANIC - its shadow passwords. Reboot the server into single user mode (type menu.c32 at the bootloader prompt, press TAB to edit the xe entry and replace "quiet vga=785 splash" with "single" then boot. At the sh# prompt, run pwconv and then echo newpassword | passwd --stdin root I use Dell DRACs which are renouned for putting in extra characters while typing. This makes blind password changes "unnreliable" Then reboot again and you should be able to connect to your server in xencenter.
If a single server, apply all the updates now like normal.
If you're a pool master, the pool will be visible again. Evacuate the next node in the pool, and do the same reinstallation/upgrade from CD again.
Once the whole pool is on 6.5 then you can install updates. Finally re-enable HA on the pool, if you want it.