Pwy is often used in Welsh to mean 'Who?'. In English 'who' is also used to introduce an adjectival clause, e.g.

I don't know the student who has won the award.

In this case, pwy is not used in Welsh. In fact, the 'who' is not translated.

Dw i ddim yn adnabod y myfyriwr sy wedi ennill y wobr.