I run Ubuntu Server 14.04.3 with ZFS on 8 3TB Drives setup in a RAID 10 Mirror.
I can tell you that KVM Performance is very good using virtio drivers for storage and network on either a .raw or .qcow2 vm file. I use writeback and lz4 compression on the file - ensure you have some sort of UPS if you're using writeback or you could lose data/corrupt your VM File in an unclean power off.
This setup works really well - if watching previously recorded video, it's fast, resilient and reliable. The only issue I have come across so far is the combination of MythTV in a VM, recording to and NFS share on the host ZFS filesystem. When watching *live*
video, or video in the process of recording - it will stutter and skip. It's just noticeable enough to be annoying and it's a reproducible issue - I attribute it to the COW (Copy on Write) nature of ZFS and accessing the file through NFS while it's being written to disk. The issue seems dramatically improved if you use Samaba on the host instead - but I still notice it when watching in progress or "live" TV.
I have not yet tried BTRFS to see if the same issue exists as I'm not ready to trust my data to BTRFS yet - but I'd welcome comments from anyone who has tried the combo of MythTV VM + Host NFS + Host BTRFS to here your experience.
ZFS is awesome - I would urge you to run mirrored vdevs over RAIDZ though - look here for the reasons as to why http://jrs-s.net/2015/02/06/zfs-you-should-use-mirror-vdevs-not-raidz/