köken: Sound shift from older クセノン (kusenon), itself a borrowing from German Xenon. Listed in some references as a borrowing from English xenon. An English derivation would have been borrowed with the initial xe- rendered as ゼ (ze) or ジ (ji), whereas クセ (kuse) and キセ (kise) instead point to Dutch or German as the source language.
köken: Borrowed from either German Xenon, French xénon or Russian ксено́н (ksenón), from Ancient Greek ξένον (xénon), neuter of ξένος (xénos, “foreign, strange”).