After beating the game go to the giant chasm. You will need strength and surf to get through and Kyurem will be deep in the cave. Check out this website for a map: http://pokemon.wikia.com/wiki/Giant_Chasm
Hope this helps :)
Kyurem hides in the Giant Chasm, you need to beat the game before you go there, you just need HM Strenght and Surf to get there
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