Why a 'V'?

[mokslininkas.livejournal.com]

Hello,

I have a question: does anyone know why in Russian it is writen kogo, chego, ego etc but is pronounced kovo, chevo, evo?
I am a native Russian speaker but never really thought much about the wonders of the lang. until now....so if anyone could please kindly explain to me why the above is the case, I'd be very greatful.....many thanks in advance.