Among the words given, austere has the least negative
connotation. By definition, it means grim or severe in manner or appearance, or
lacking trivial decoration or not extravagant or gaudy. Both severe and harsh
are rather negative in connotation. Severe means very bad, intense or strict.
Harsh suggests a nature that is unfeeling, cruel, and indifferent to the pain
it inflicts. If applied to things, harsh implies effectiveness in promoting
discomforts or in imposing rigors.
I hope this helps.
