Pretty sure it's short for "Pura Vida." Jack Dorsey is hanging out in Costa Rica, where the phrase is used in reference to a peaceful and happy lifestyle. "Pura Vida" is also attached to one of his relays, further supporting the idea.

So saying PV is the ultra short version of something like "I hope life provides you with continued happiness." At least that's how I'm reading it.

PV Mutxo