Should I still use "Dear Prof." to begin an email to a professor that I know? Let Smith be the last name of a U.S.-grown-up professor you not only have taken part in whose class but also ended up becoming a friend of whom in a general sense; I wonder if it still sounds good if in an email to him one uses "Dear Prof. Smith" to address him. I am not really sure, given this condition, if using "Dear Prof." to begin an email to him would somehow become a tone that he does not like? But, on the other hand, I am afraid using, say "Hi Prof. Smith", would sound like one is exalting himself to the place at which Smith stands as a professional scholar. 

The title of this question may not be precise (I am not an English native speaker); please feel free to edit it.
In the US, it is very unusual to combine the title "Professor" with a first name. I have never been referred to as "Professor Tom" in my whole career.

You can either write "Dear Prof. Smith", using the family name, or "Dear John". You should only write "Dear John" if you would call him "John" when you speak in person. (For people you haven't met in person, the decision is a bit different; but for people who you know personally, this is a good rule.) For example, my students generally call me "Prof. Church", but sometimes after they graduate I invite them to call me Tom; in this case, it would be perfectly appropriate to send me a letter beginning "Dear Tom".

I would use "Hi John" only to convey a very informal tone. Even with colleagues or students with whom I'm good friends outside of work, I still use "Dear Mary" when contacting them about professional business. For this reason, I don't recommend using "Hi Prof. Smith"; it mixes an informal term with a formal term.