Are most of the NoSQL databases key/value stores technically?

This video distinguishes between three (main) types of NoSQL databases: 1. Key/Value store 2. Tabular 3. Document oriented Do these types differ in how they technically store their data or can they be considered all to be key/value stores since even the tabular ones (like Cassandra) may have variable column count and most likely serialize the column values to a string? And the documents (MongoDB) are also only strings in the end... I suppose all types generate an unique id for their "values/data". So are all the 3 types key/value on low level?