A twenty year old virgin. Someone who has reached the age of 20 without having penetrating sex (oral sex doesn't count as losing virginity).
The double 'x' is taken from the Roman numeral for 20. YOV is an acronym for "year old virgin". Perhaps originating from the film "40 year old Virgin" staring Steve Carell.
No xxyov's known to exist, but scientists are searching for any clues pointing to a previous existence.