From the Oxford English Dictionary:
taxi, v.
1. a. intr. Of an aeroplane, etc., or its pilot: to travel slowly along the ground or water under the machine's own power. Also transf. to taxi in, to taxi from a runway to a terminal or hangar; similarly to taxi out.
b. trans. To cause (an aeroplane, etc.) to taxi.
"Taxy" is mentioned as a spelling variant of taxi, but only for the present participle (i.e. "taxying"). There is no separate entry in the OED for "taxy".
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