An asphixiant is any gas that is none toxic but will kill you if you try to breath 100% of it because it will starve you of Oxygen.
Yes. While non-toxic, if it displaces enough oxygen suffocation will occur.
Xenon is a noble gas, and provides an inert atmosphere for most chemical reactions--it does form fluorides and oxides, however, the latter of which are sensitive explosives and can oxidize metals to their highest states. Xenon is also a medical asphixiant.