Perv is short for pervert. Pervert means some one who is interested in seeing people naked or having sex with them. We are all perverts, we are just afraid to admit it.
(n) (1) Someone perceived by the speaker as being sexually obsessed or promiscuous (rarely a true pervert)
(2) A retort one uses when they think the other person is being twisted in some fashion
(1) Your little brother asked me if my breasts were real. He's such a perv.