'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.