I'm Russian so I'm going to say it like this, Pavel is Russian its not a translation into English from Pasha. Pasha is just another way of saying Pavel, lengthwise it's the same, but Pavel and Pasha are the same name also.
Actually pasha is pavel in Russian. It's not short, it's the exact same length.
It's like Sam and Sammy, Sammy isn't short for Sam, but a softer way of saying Sam.