Guests are unwelcome. Is it passive or active? The verb here is tranisitive and the guests aren't the one that unwelcomes.
In your sentence, unwelcome is definitely an adjective (there is no such verb as unwelcome), functioning as the subject complement.
A sentence with a passive voice would be
The guests are welcomed by ...
as both past and past participle of welcome is (or are!) welcomed.
“Are’ is the verb used in this sentence.Hence this sentence belongs to Present Tense.
It is not in:
Continuous tense, prefect tense or perfect continuous tense.Hence it is in Simple Present Tense and in affirmative form.
Formula for this sentence in Passive voice is as follow:
Subject+ helping verb + Past participle.+................
Note: This is no past participle form of the verb ‘unwelcome’.
Hence it is not in passive voice.