He was born in a small town called Kuruhur, in the southernmost region of the Tamil country Tirunelveli. His father, Kari, was a petty prince who paid tribute to the Pandyan King of Madura.

   He must have been born fully enlightened because as a baby he never cried or suckled and never opened his eyes. According to tradition, as a child he responded to no external stimuli and his parents left him at the feet of the Deity of Lord Vishnu. The child then got up and climbed into a hole in a tamarind, sat in the lotus position, and began to meditate. It appears he was in this state for as long as sixteen years when a Tamil poet and scholar in North India named Madhurakavi Alvar saw a bright light shining to the south, and followed it until he reached Nammalvar's tree. Unable to elicit any reaction from the child, he asked him a riddle: "If the small is born in a dead's body(or stomach), what will it eat and where will it stay?" meaning, if the subtle soul is embodied in the gross body, what are its actions and thoughts? Nammalvar broke his lifelong silence and responded, "That it will eat, and there it will rest!" meaning that if the soul identifies with the body, it will be the body but if it serves the divine, it will stay in vaikunta and eat(think) of God. Madhura-kavi realized the divinity of this child.

Source : http://en.wikipedia.org/wiki/Nammalvar