I can't find Kivan since I kicked him out of my party back at High hedge!

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"No!" shreiked Tyrion. Though the sword felt heavy as a fallen tree he lifted it. He saw the deamon look up, astonished. He brought down the blade with the awesome majestic power of a falling thunderbolt. The burning blade caught the deamon right in the centre of its forehead. The deamon's head fell apart under the force of the impact. The deamon N'kari sank to its knees, dead.

-Tyrion at the battle of the Blighted Isle.

I'm not sure on this, but I remember someone saying that if you drop Kivan from your party he will never come back.

I doubled checked in a BG1 guide I found on gamefaqs.com and unfortunately Kivan will leave for good if you dismiss him. He probably goes off to revenge his lost love or return to his homeland..

If you are really depserate to have Kivan back and you're not averse to cheating in this sort of instance you can easily CLUA him back into the game. (If you don't know how to do that and want to know how let me know and I will post it.) But living with consequences learning from mistakes and is part of the game. Lesson: always drop NPCs indoors.