For those who has tried backup Hyper V using DPM 2010, you will need to select different protection group server. Here is the snapshot that you can refer to
a) Clustered Hyper V host.
Select cluster name rather than individual server name. For it own server name, you only can see Initial Store.xml rather than the entire Hyper V VM.
b) Non-cluster Hyper V host
Select individual server name.
Make sense ! Right..