Just some notes, in case others are planning similar upgrades.
I was running 2.6.32 from M. Young's repo. My goal was to get up to 2.6.40 on Fedora 15 (mainline dom0!) with minimal downtime.
I found that f15's version of Xen (4.1) wouldn't run, even after re-compiling on f14. So, that couldn't be staged ahead of time. As a retreat plan, I made sure to have rpm's for Xen 4.0.2 handy, but they wound up being unnecessary.
I knew there'd be some xend breakage during the upgrade, so I opened ssh sessions to each VM.
I then started the yum upgrade, first with yum and rpm, then only the fedora and updates repos, then other repos. Some package upgrade problems needed to be solved (I have too much in dom0), but nothing worse than usual.
Once all the upgrades were in place, I hand-edited grub.conf since the new kernel installed a non-Xen entry.
At this point, I should have checked to see that my initramfs was made properly (it wasn't, locally packaged zfs modules broke the scriptlet). I had to come back in with rescue mode to re-run dracut.
Once all the updates were installed, I shut down each of the VM's using native 'shutdown -h now' inside the vm. xm shutdown was broken at this point.
Now, the only ugly part is that systemd wasn't yet running, but 'reboot' is replaced. So, to reboot the system, I had to do 'sync;sync;sync [reset button]'. I had filesystems mounted -o data=journal, so I was at least feeling good about consistency. Some people wouldn't tolerate this option, but it worked out OK for me.
With a working initramfs, the system came up just fine, the VM's started as they should, and everything seemed OK from the Xen perspective. All the domU's function, no domU updates required (Fedora 12/13/14, CentOS 5).
If I had noticed the initramfs problem properly, I would have gotten away with only about a 4 minute outage. Quite glad to be re-united, after many years in the Fedora/Xen exile community! Much kudos to Michael Young, Pasi, and all the others who have helped up through the dark times.
-Bill