Xuanzang is a special monk. He's a reincarnation of a "godly" being.
Along the journey, Xuanzang is constantly terrorised by monsters and demons because of a legend which says that one can attain immortality by consuming his flesh because he is a reincarnation of a holy being.
Wikipedia - Journey to the West, Xuanzang
Basically, it's rumored that by eating Xuanzang, a reincarnated holy being's, flesh, demons themselves can become immortal.
San Zang was originally Jin Chan Zi, Buddha's disciple. He was banished to earth because he was disobedient, and didn't pay attention to Buddha's teachings. He was reincarnated as Mr. (Guang Rui) and Mrs. Chen's son.
(Different versions of stories follow, but to summarise:)
Xuanzang was abandoned by his mother, after his father was killed by a man, who stole his identity and wife. The baby was set afloat in a river, and found by a priest, who later raised Xuanzang as a monk.
Years after that, when Xuan Zang was all grown up, the Emperor of Great Tang asked him to hold a religious ceremony. On the ceremony, Xuan Zang was approached by the disguised Goddess of Mercy and her disciple. They gave him a monk's staff and robe and told him to fetch the scriptures from the Western Heavens.