Im pretty sure its skin to skin contact.

The types of hpv that infect the genital area are spread primarily through genital contact. Most hpv infections have no signs or symptoms; therefore, most infected persons are unaware they are infected, yet they can transmit the virus to a sex partner. Rarely, a pregnant woman can pass hpv to her baby during vaginal delivery. A baby that is exposed to hpv very rarely develops warts in the throat or voice box.

I got that info from: http://www.Cdc.Gov/std/hpv/stdfact-hpv.Htm #howget