My understanding is that all rewards from a Care Package do not, nor those from Emergency Airdrop crates. Was the Pave Low recently changed to count? What about the Stealth Bomber, Precision Airstrike, Attack Helicopter and the Sentry Gun?
As far as I know, Pavelow has always counted. Only care package drops/sentry guns don't count and any killstreaks that you earned in a previous life do not count. All other killstreaks count towards a higher killstreak as long as you don't die.
That is, if I were to earn Pavelow via a 9 kill streak and then die, the kills that the Pavelow will get will not count towards a nuke or EMP.
There used to be a glitch surrounding the Pave Low. Before, only the first Pave Low called out would contribute to you kill streak (and then only sometimes). I believe this behavior was patched and it acts like a normal kill streak now. You are right about care packages and emergency airdrops though. However, I believe if you get a crushing kill by dropping the care package on someone, that does count (not that that happens very often).
All killstreak rewards that you control count towards your streak. You have the rewards that you can control for 100% , like the Chopper Gunner, AC130, Predator Missile. And you have the rewards where you choose where to place it, like the Harrier Strike, Predator Strike, Stealth Bomber
I'm unsure about Pavelow: with the glitch it counted towards you streak whenever you got one kill AFTER gettin the Pavelow (before calling it in). If this is fixed I don't know how it works now.