i'm not sure what the exact difference is other than they are short finned, have less than 4 rays, are usually dragon scaled, and should have an asymmetric anal fin. They also have stockier bodies and dont normally reach 180* and tend to have darker or more wild type colorations. Plakats are usually bred as fighters and HMPK is bred as a show or pet type fish. (not saying everyone that breeds for PK is breeding to fight, but by the way I see them classified (5*) makes me think that its pretty common)
Really, the only real difference between PK and HMPK is plakat is the original form and HMPK is a HM x PK mix so they have short fins, but look more HM than a regular PK.
See the difference in their Caudal and anal?
Also, I hope that what I said made sense, its been a longggg weekend!