There is only one file type that uses the .KZ extension:
KuaiZip Compressed File
A KZ file is a compressed archive created by KuaiZip, a free file compression and decompression utility. It is compressed using a proprietary format that can increase the compression ratio five to thirty percent over standard .RAR compression and ten percent over .7Z compression.
KuaiZip is a Chinese program and is not localized in English, making it difficult for non-Chinese speakers to use. Therefore, you most likely will only use the program if you receive a compressed KZ file from a friend or coworker and need to extract the contents from the archive.
Updated: January 22, 2020