FIT is binary, so map tools and text editors cannot read it. KMZ bundles a map with its icons and images in one file. FIT is at home in workouts recorded by sports watches and bike computers; KMZ is the usual choice for KML with its icons and overlay images in one file.
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. Only recordings with GPS positions can be converted; an indoor workout with no route is refused. Time and elevation are kept. Heart rate, cadence and power are not written, because KMZ has no standard place for them; choose GeoJSON or CSV to keep them. Altitude and per-point timestamps are kept; a line with timestamps is written as a Google Earth track that can be played back. The KML is zipped as doc.kml, the name Google Earth expects.