for my KVM guests I use only LVM disks, I find it easy to use and administer.
Every time you running out of space you can resize your lvn volume.
Bus some days ago I moved some virtual servers to another physical Dom0(Host) and was a little bit confused – how to move lvm – DD was not the best idea.

There is always another way, here is my:
Very important! Shut-down virtual machine.
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