Pokebank won't allow Black/White Kyurem to be transferred to Pokemon X or Pokemon Y, so Black/White Kyurem has to be split into Kyurem and Reshiram/Zekrom before you transfer it.
Source: http://pokemondb.net/pokebase/183080/why-cant-i-pokemover-white-or-black-kyurem-into-pokemon-bank