They basically mean the same thing because if you've never been to Korea, then you haven't been there. On another note, if you change the question and specify the time and say something like "Have you been to Korea this year?", then it will make a difference.

The first two mean the same thing and are completely interchangeable. The only "differences" that I can think of is if you specify a length of time, as mentioned by Dennisse. In that case, the added "ever" would be incorrect, and you would have to drop it to say "Have you been to Korea this summer?" The "ever" implies that you're asking about if they have been at any time in their lifespan, and would conflict with the specific time frame that you're asking about. As for 'I have never been to Korea.' vs " I have not been to Korea,' these follow the same rules as above: If you leave the phrases as it they are, they mean the exact same thing and are interchangeable. However, if you're asking about a specific time frame, the "I have never been" will be incorrect and you should say "I have not been to Korea this summer" (instead of "I have never been to Korea this summer."). I hope that helps.

Oh, my..silly mistake..I think first I tried to write 'what is the difference between two sentences? and then changed the word 'difference' as 'differences' because I added two more sentences without noticing that I also had to change the verb.

What (are) the differences? Not, "What is the differences." Because "differences" is plural, you need to use the plural form of to be, which is "are." Difference = is DifferenceS = are For example... The DIFFERENCE between us IS huge, but the DIFFERENCES between you and him ARE small.