Well, I have always thought about it. I am a 37 y/o man and for me the difference is like this:
Pretty: has to do more with your face and your facial characteristics like your eyes, your nose or your mouth, or everything together. For example, you may be a little chubby yet you may have a very nice face with big round eyes or nice straight nose and mouth. You would be pretty.
Hot: I think hot and sexy are pretty much the same, but in my view, you are hot when you have a real nice body and/or you are very physically attractive (according to the western and present concept of "nice" and "attractive"). For example, if you're tall and skinny yet you have large boobs and round firm *** or long legs, then you would be hot. I think you can be hot but not pretty, if you have a nice body yet a not-so-nice face.
Sexy: I think this has to do more with the looks IN COMBINATION with the attitude, not necessarily with the physicals. For example, if you're kind of average (not so perfect body, or not so big boobs or big *** or whatever) yet you dress up in attractive clothing (like miniskirts or blouses with cleavage, or tight jeans and high-heel shoes) PLUS you look confident and you act confident, then you would be sexy.
I know many girls who, in my view, are very sexy yet not very pretty. I also now some girls who are extremely pretty, yet not sexy at all.
I think men like different things at different times of their lives. When we are young a pretty girl may draw our attention. Maybe when we are young adults we interested in girls who look hot or sexy.
This is just my view. Some guys may think differently. There are no rules for any of the definitions after all.
I think, that, if you are pretty and you can add some sexy to it by actually having a sexy attitude (plus the clothing and look), you would be extremely attractive to men. Just be careful with the looks. If you go overboard on the clothing, shoes and makeup, you may cross the line between "sexy" and "foxy".