HIV mutates almost every time a new copy is made, but not every mutation causes resistance. Anything different from the wild type is considered a mutation.
Antiviral drugs control most types of the virus.
However, virus that is resistant to a drug you are taking will multiply and become the most common form in your body. If you stop taking antiviral medications, there is no selective pressure. The wild type virus will multiply the fastest. We don't know whether drug resistance can disappear, or whether it will come back if you re-start the same drugs. Research studies have shown that resistance testing helps doctors make better treatment decisions for their patients.