The etymology of "chav" is debated. IMHO it probably does not derive from the Romany "chavi".

The usage also varies.

Amidst the musically minded young it denotes followers of mainstream pop music rather than any of the "alternative" (yes, I know that that has specific overtones but it is a useful catch-all) musics, who as well as dressing with execrable taste threaten and assault followers of minority musics. There it probably has less attachment to education or intelligence (or lack thereof).

In the mainstream indicators would be - label driven clothing, bling, threatening behaviour, poor vocabulary speech and manners, loitering in groups on street corners or in bus shelters, drinking strong chemical cider to nausea, casual theft and violence, distaste for effort or self improvement or education or proper speech. Think Vicky Pollard from the TV comedy "Little Britain".