OK . . . it's quite a subtle distinction. Firstly, it must be pointed out that in most cases, "I know" is used on its own without an object pronoun. "I know it" would be used only if it relates to knowing a fact of some sort, ie "who knows the answer to question 5?" -- "I know it." "I know that" would be used if the object is more abstract, ie "taking drugs is dangerous" -- "I know that, but it's fun". Let me know if you need more examples, it's kind of difficult to define.
As ravenhair says, these are very subtle differences, but with "I know it" the stress is generally being laid on the "I". Ravenhair gives one example; another would be if several people were asked "who knows the way to the pub?" "I know it" (emphasising "I"). On the other hand in "I know that" the stress is often being laid on "that", as in ravenhair's example or in "do you know you're putting on weight?" "I know THAT!" I hope this is of help.
if you say ' i know that' itr would mean thats 'that' would be something specific, and it sounds better lol, if you say 'i know it' personally i don't think it sounds gramatically correct.