What are some french words that start with a letter x?

There aren't too many:

xénogreffe (xenotransplant)

xénon (xenon)

xénophobe (xenophobe or xenophobic)

xénophobie (xenophobia)

Xénophone (Xenophone)

xénotransplantation (xenotransplantation)

xérès (sherry)

xérophyte (xerophyte)

Xérès (Xeres)

XML (same as in English: eXtensible Markup Language)

xylograph (xylographer)

xylographie ([the art of] xylography or [a single] xylograph)

xylographique (xylographic)

xylophage (xylophagous)

xylophene (wood preservative)

xylophone (xylophone)

There may be more in an unabridged dictionary, but those are the only ones listed in my Oxford-Hechette bi-lingual one.