Though Apollo Xenios is attested (on the island of Chios), this is, par excellence, the epithet of Zeus. See in brief W.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.).