KMZ is a compressed archive, so text editors and many scripts cannot read it directly. KML opens in Google Earth and Google My Maps. KMZ is at home in KML with its icons and overlay images in one file; KML is the usual choice for places, routes and tours viewed in Google Earth.
Every format is read into plain GeoJSON features and written out from there, so coordinates, names and properties carry over wherever the target has a place for them. XML that declares custom entities is refused, and KMZ files are parsed without touching the network. The KML inside the KMZ is read; icons and overlay images in the archive are not carried over. Altitude and per-point timestamps are kept; a line with timestamps is written as a Google Earth track that can be played back.