CSV carries no formatting, formulas or multiple sheets, and Excel often misreads its dates and encodings. KMZ bundles a map with its icons and images in one file. CSV is at home in moving tabular data between systems; 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. The first row must name a latitude and a longitude column (lat, latitude, lon, lng, longitude and similar); name, elevation and time columns are recognised, and every other column becomes a property. A row with no valid coordinates stops the conversion and is named. 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. Coordinates outside the latitude and longitude range are refused, since the file is probably in a projected coordinate system.