KLV (Key-Length-Value) is a data encoding standard, often used to embed information in video feeds.
It is defined in SMPTE 336M-2007 (Data Encoding Protocol Using Key-Length Value), approved by the Society of Motion Picture and Television Engineers.
Due to KLV's large degree of interoperability, it has also been adopted by the Motion Imagery Standards Board.
In a binary stream of data, a KLV set is broken down in the following fashion, with all integer-interpretation being big endian: The first few bytes are the Key, much like a key in a standard hash table data structure.
The remaining bytes are the Value field, and its contents can be whatever you like, including a chain of more KLV sets, as is often the case.