I don't know if that stat would cover more than one round, but I cannot imagine playing a full round of BF2 without even dieing at least once ( with the exception of being commander maybe but even those die a little when enemy Spec Ops atack the base ). So I'd guess it is per round. chuy may answer this more precisely though.
Playing F-15 pilot really helps to raise the kill streak if there are no good pilots in the other team and you have enough time ^^
Else commander is a good way as arty kills count towards the kill streak. Got my 26 during a round of Sharqui Peninsula yesterday as most of the MEC were running around the most western flag.