None of these words is really much more formal than the other, but they are used in different contexts. "Well-known" means that someone or something is very familiar to a group of people. Ex. "The man with the yellow hat and his monkey george are well-known in this small community" "famous" describes someones or somethings popularity among a group of people. Ex. "The fried chicken is famous in this town!" "celebrity" is a noun that describes a person with exceeding popularity and they are usually known by an entire country or the world. You can say "local celebrity" to talk about a person who has reached such a status in a smaller community. Ex. "As a celebrity, he gets all the praise from the media and his fans." I hope this description helps. If you are still unsure, please ask.

Well known usually refers to a smaller circle, or people in a particular situation, but still known by many. Famous can mean someone or something and means that a lot of people know their/its name or have heard about them/it. Celebrity is usually used to reference a famous person. Some examples are: "This hotel is know for its celebrity guests." "Neil deGrasse Tyson is a well known astrophysicist." "Kelly Clarkson became famous after appearing on American Idol."