«Καλώς» is an adverb and means "well". Although it's absolutely acceptable as a term, this way of writing and speaking refers to an older way of using the language. «Καλά» originally is an adjective (neutral, plural) and means "good" (e.g. Τα καλά παιδιά, the good kids). But, contemporary grammar accepts «καλά», and other words with this ending, as adverbs as well. So, it's absolutely correct, and in fact way more common, to use «καλά» as an adverb.
- Τι κάνεις; (How are you)
- Είμαι καλά. (I am well)