I don't really understand why in the world it's even used. Acronyms like 'lol' and such are generally used to shorten keystrokes, but 'pwn' and 'own' have the same number of keystrokes. Besides, it should be much easier to type 'own' (since it's an actual word that should be more familiar to you, but also because I think it's easier to type 'o' with my ring finger than type 'p' with my pinky). Even if you're verbalizing it, 'own' is still shorter than 'pawn' or 'pown.'
Back on-topic, I mentally pronounce it 'pawn.' I suppose I'm a more phonetic person, so I go with what it looks like.