So I'm using Addictive Drums and I've got a track with midi loops from different addpaks. The beats sound good together, rhythmically but there is some not so subtle variance in the volume and velocity when transitioning from one part to the next. What's the best way to even this out. With audio I would use clip gain. Thanks!

Maybe MIDI compression? Check out this thread...
 
http://forum.cakewalk.com/MIDI-Compressor-anybody-m3437338.aspx
 

There is also the velocity scale function, not sure if it would do what you want but it has a low and hi limit so would take down any higher velocities. 
 
What I also have done is highlight the whole clip and drag the velocity up until a bunch hit the top and there is still some dynamics showing. Let go. now do this again and drag them back down a bit. What this will do is take all the notes that were say between 100 and 128 and make them all 100. 

Hmmm... I'm starting to wonder if I'd be better of using the beat transforming part within addictive itself.

I'd separate each kit piece into it's own MIDI track.
Open the Kick MIDI track in PRV, highlight each clip, go to the Velocity pane, and drag the performance until it's playing where it matches the others.  Then do the same with Snare, HiHat, Toms, etc.
Then bounce them all back to one MIDI track for simplicity.

Enabling "Show Velocity on Selected Notes only" in the PRV Notes menu may make it possible to avoid the need to split and recombine the MIDI data.