What is the difference between avoidance therapy and aversion therapy?

Technically, there is no such thing as "avoidance therapy." However, avoidance learning is a general term referring to any situation where the correct response allows an organism to escape a negative outcome.

There is also something called avoidance motivation, which basically means taking or not taking action to avoid a negative outcome. For example, a guy is really interested in getting to know a girl he likes. She's good looking, kind, and intelligent. He doesn't know her very well. He's a little shy and has a hard time approaching people. He really likes this girl, but there is a problem. He believes he will get rejected, make a fool of himself, or do something ridiculous. As such, he never approaches the girl. His avoidance strategy is to just never try.

Aversion therapy on the other hand, is a type of behavioral therapy that uses positive punishment to reduce the frequency of an undesirable behavior. For example, Alcoholism is sometimes treated with a drug called Antabuse, which increases the patient's sensitivity to alcohol so that drinking even a small amount leads to a very intense and unpleasant physical reaction such as nausea or headache. Although aversive therapies are sometimes useful for controlling an unwanted behavior over the short term, in many cases, they are not particularly useful for long-term change.