Our Issue with commvault was that despite it mounts the snapshots in the proxy VM it has to read them as a whole. So Inc Backups take nearly as long as Full backups. When you want to backup 20TB+ Incs every night, Thats not acceptable. So the Lack of CBT is really a big deal when doing VM Backups.
Performance with CommVault would be not that big of a problem since you can do multiple streams and multiple VMs simultanously but we could not get past 600MB/s which is a Xen Problem as of our Testing. With HyperV we see Performance in access of 1.6GB/s in the same Scenarios. But also this performance would be much to slow to do Incs every night.
We tested XO CBT but it was not stable and for what i read in the posts there seems to be problems with CBT and Live-Migration so that the CBT state seems to be lost. Also a big Problem for us.
When we used CommVault to backup multiple VMs we ran into Blue-Screens of the Xen Toolstack in the Windows VMs when attaching or detaching Snapshot-VDIs to the Proxy VMs. There clearly is a bug in the Xen Windows Drivers. So we had to reduce concurrency which reduced backup speed.
We didn't do a short test with xen, we migrated out internal production Cluster to it from VMWare and used it for about 4 month now (30+ VMs on iSCSI Storage with 3 Nodes) and after we ran into more and more Problems we had many discussions with our team and we had to ask ourselfes what Xen brings to the table that is worth the drawbacks. The only scenario where it fits for us is where we have to map physical hardware into VMs.
We decided to come back in some time to see how the SMAPIv3 drivers work out and if there is a better support for shared storage.