Facebook's PAR is a self-extracting zip file, I assume Google's is similar. XARs are self-mounting SquashFS archives (a compressed read only filesystem). This means that XARs don't have to be extracted to a temporary directory to run, they can run in place. Zip files have to be completely extracted before running, but SquashFS decompresses pages on the fly, so startup times are much faster (especially with zstd compression).
