Though Apollo Xenios is attested (on the island of Chios), this is, par excellence, the epithet of Zeus. See in brief Walter Burkert, Greek Religion (Cambridge Mass. 1985) 130 ("Zeus has a special concern for the relations which bind strangers to one another: guests, suppliants, and those bound by oaths - Zeus, Xenios, Hikesios, and Horkios") and 248 ("Zeus guards the laws of hospitality in the domain of house and court, and over strangers and suppliants who have arrived in the protecting domain: Zeus Herkeios, Hikesios, Xenios").
See again xi 57 (and under mu 236, omicron 251).
[1] "Protector of the rights of hospitality" (LSJ s.v.).