KML is XML tied to Google Earth's way of styling, and not all GIS tools read every feature. WKT is what spatial databases and GIS libraries accept as text. KML is at home in places, routes and tours viewed in Google Earth; WKT is the usual choice for writing a single geometry in a database or script.
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 KML files are parsed without touching the network. WKT describes shapes only, so names and properties are left out; each feature is written on its own line.