We usually write:
But if I'd like to write simply Hi followed by a Period, ie,
How are you?
Is it correct?
"The reader's ability to comprehend the logic included within their written expression and the reader's ability to read the writing aloud with proper vocal inflections and cadence is what is important" as said by whackademic. Both your sentence can be used if the punctuation helps a person in understanding what was inferred to him/her.
Hi used in different types of sentences,
You might also want to take a look at Hello which is similar to your question and also all the answers there can be applicable to 'Hi' also.