Microsoft just confirmed that this is a change in DPM 2012 R2 U3 when using Hyper-V on 2012 R2. They don't use the hw provider anymore. It is not documented in technet yet.
Hi, I installed a Hyper-V with DPM in April and made shure the DPM makes backups of the virtual machines by creating SAN snapshots through the VSS hardware provider. Sometime in august it suddenly stopped using the VSS hardware provider. I changed a lot since april, installed a Cluster, made the volumes as CSV, installed VMM. I did not find any error messages about the VSS provider. The dpmra logs finds the vss hardware provider. The backups are still working but only with software snapshots. I can make VM snapshots of the affected VMs in ASM without issues. So the VSS hardware provider still works. I did not changed the SAN firmware (7.0.5) or the HIT (4.7.0) since it worked. I just opened a case with Microsoft, but if anyone have a clue what I could have changed or where I should look...
2012 R2 Hyper-V and DPM not using SAN Snapshots anymore