‘I’m’ is merely a contraction of ‘I am’.

From Wikipedia:

A contraction is a shortened version of the written and spoken forms of a word, syllable, or word group, created by omission of internal letters (actually, sounds). In traditional grammar, contraction can denote the formation of a new word from one word or a group of words, for example, by elision. This often occurs in rendering a common sequence of words or, as in French, in maintaining a flowing sound.

‘I’m’ is always used in conjunction with a noun phrase. You cannot write “A boy, I’m”, but you can write “A boy, I am”. ‘I’m’ may also be considered informal outside speech or a literary scope.

‘I am’ is also longer to pronounce, and therefore has more emphasis (as pointed out by one of the answers).