'Hello' and 'hallo' are the same word. 'Hallo' is simply an alternative way of writing 'hello'. It's quite old-fashioned and rarely used these days. Nowadays, almost everyone writes it 'hello'. German has an equivalent word 'Hallo', by the way, which is used the same way. 'Hi' is a more casual and informal version of 'Hello'. In the past, it was mainly an American greeting, but now it is used worldwide.

the only difference between "hi" and "hello" is that "hi" is more of a casual greeting. as for hallo... iv actually never seen hallo... learn something everyday i guess! haha.

Hi Hallo