It might have some things in common. In my case, the objects are persistent and can be read to and written from disk with ease. The relationship between keys and values can be 1:1, 1:many, many:1, or many:many.

If the KV store is a 'state' column in a DB table of US customer addresses for example, the number of unique values is limited to 50 (if you ignore D.C., Puerto Rico, etc.). The table might have 100M customers, each mapped to at least one state. Each state (except less populated ones like Wyoming) might have millions of rows mapped to it. Many customers might have addresses in more than one state.

The system has to do things like: "Find every key mapped to a state that starts with the letter 'M'" or "Find every value that is mapped to this set of keys". These are the kinds of things a normal SQL query has to do.