The IV's are determined at Random when you receive the Event Pokémon, unless of course the Event Pokémon in question has fixed IV's.
If you want to try and get an Event Pokémon with better IV's you could always save the game before the delivery man gives you your Event Pokémon, that way if you don't get the IV's you want you can Soft Reset by pressing and holding L, R, and Start or Select. This is the same case with Natures etc.
