Is hot an adverb

No. But it can seem to be, where it refers obliquely to an action, rather than a noun.

In the sentence "The food is hot" the Word hot is an adjective describing food.

In the sentence "The food should be served hot" the Word hot seems to be an adverb, indicating the manner in which the food is served - many dictionaries classify it as an adverb in these cases. However, it is a condition, not a manner; it is not the serving that is hot, but the food.

The actual adverb is "hotly" and it is used to describe the manner (often metaphorical) of an action, e.g. The sun shone hotly on the rocky plain. The election was hotly contested by the two factions.