Comes from a police term:
"Council House Associated Violence". Basically anything that's gained or has the potential to gain an Anti-Social Behaviour Order. Too young to prosecute fully, too old for a plain and simple slap - unless you know somewhere really dark you can lure him to first >;¬)
A 14 year old burberry and "bling" clad vandal/gangster wannabe who belongs to parents that are probably too busy to pay attention to little "Kev" except when the police knock the door because he's TWOC'rd next door's car again , causing the likelihood of an ASBO and them all getting evicted.
Kevchav has a 13 year old girlfriend called Chantelle who's pregnant, yet they still spend their pocket money on carlsberg, fags and chewing gum.