Robert is right. Many people with a typical accent of London, and some other English cities, pronounce the 'th' sound in 'brother' as a 'v'. So 'bruv' is an informal version of 'brother'. Like the American 'bro', I guess. And yes, and 'x' is a kiss. It's the standard affectionate sign-off for any letter or message between friends or family.

I'm not English, but I think it's a British way of saying "brother", and the X = a kiss. X X = Kiss, kiss. O, O = Hug, hug XOXO = hugs and kisses