This link should help
http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_feature_guide09186a0080110bc5.html#1146054
Each pvdm that you insert into a nm-hdv has 3 dsp chips. You can insert 5 such pvdms into a Nm-hdv.
the nm-hdv-farm-c36 has is a nm-hdv which comes with 2 pvdms pre installed.
nm-hdv-farm-c54 has 3 pvdms
nm-hdv-farm-c90 has 5 pvdms
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus