"Hold it" has a more forceful tone. You can use "hold on" in cases where you would use "hold it," but it's not a good idea to use "hold it" when you would use "hold on." If you and a friend agree to go out to do something, but you have to do something else beforehand, you would us "hold on." If you use "hold it," you give a tone that your friend is not supposed to be leaving.

this is a tough question. To answer what you are asking, I believe hold it and hold on are different in that "hold it" would be to stop what you are doing indefinitely, while "hold on" would mean to wait or pause what you are doing. I personally wouldn't use hold it, but this could be a cultural saying. I am from the USA, not sure if that makes a difference.

I like to ask your question, hold it, like an order, verb+ object, hold on, means stop at a point, when you are listening someone's talking, and you want to ask a question, of course you say hold on, you need him or her has a stop. Same as I ask him to stop talking....... I ask for a help, ,,,,, verb+ object and verb+proposition, ,,,,,,,How about my explaining?