A difference between software transactional memory $($STM$)$ and hardware transactional memory $($HTM$)$ is that

In HTM$,$ code is inserted by a compiler, while in STM$,$ user enters the appropriate code.

HTM requires existing cache hierarchies and cache coherency protocols be modified, while STM does not.

STM requires no special code instrumentation and thus has less overhead than HTM$.$

HTM uses cache hierarchy and cache coherency protocols while STM uses software implementation in addition to the cache hierarchy and cache coherency protocols.