Student or Learner
What is the difference beween saying just "Hi" from "Hi there"?
Not much difference at all in AmE. Sometimes "hi there" is preferred when greeting someone from a distance (for example, from across a large room or maybe when on TV and greeting someone in the viewing audience). And "hi there" might be used when greeting a friend you haven't seen in quite a while: "Well hi there, how have you been?" But for the most part the two greetings are interchangeable.