I was wondering how PvP affects my char's experience, as i didn't notice any changes by killing serco after serco, but loosing serco's sympathy ;-)

pvp will (if you are in certian sectors) make you loose standing with that nation. Unless the person is hated or kill on sight with the nation.

You also get some combat exp.

Does the amount of combat exp. depends on the levels of the other player and if yes how ? (Someone told me you loose exp. by killing less experienced players)

yes is a player has 0/0/0/0 you lose some combat exp, and the higher level a person is the more exp you get from them when you kill them.

Actually you can lose exp for killing higher level characters than just 0/0/0/0. I killed a player in a Rag in grey space, and lost 400 XP. I think this is one situation where you shouldn't lose exp though I think this has been posted on elsewhere (I am Itani and he was Serco also).

Actually I should note that there is a warning after you first damage a lower level character, but it can be difficult to notice in the heat of combat, and if it's on the other chat tab.