Keyvi is a key value index built using finite state machines, making it an immutable key value store. It was developed as a drop-in replacement for Redis to serve as the backend search index for Cliqz, allowing them to reduce data size and machine requirements. Keyvi provides faster performance than Redis through its single-threaded, shared memory model and ability to perform auto-complete matching, approximate matching, and scoring-based matching through techniques like Levenshtein distance.