This is a matter of semantics - how much needs to be changed to stop calling it a cathinone? As cathinone is not a technical name, it's hard to say!
Methylone is a cathinone. (right everyone?)
Is MDPPP (methylone with a pyrolidine instead of the methylamino group) a cathinone? I'd find it hard to say no, as you just have a subsituent on the amine that gets named differently.
Is butylone a cathinone? Is buphedrone? Both of those have a longer carbon chain, but most people would consider them cathinones.
If the answer to both of those is yes, than you'd have to call MDPV a cathinone, whereas if you answer no to either of those, MDPV is not a cathinone.
On a more practical sense, they're all beta ketones, are active with the primary activity being that of a stimulant, they're generally of fairly similar durations (probably because the dominant metabolic path is reduction to the centrally inactive alcohol), and have broadly similar properties. I question whether it's meaningful to draw a line down the middle of these chemicals and say "these are cathinones, these are pyrovalerones, and they're different classes of drug".