"Similarity" is a noun. In that sentence, it functions as the head of the subject (the "notional" or "real" subject). The "existential there" is the formal/grammatical subject of the sentence, "is" is the head of the predicate, "little similarity" is the real subject, and "between a limousine and a Volkswagen" is what some authors call an "adverbial complement" (instead of "subject complement" since the construction is not nominal or adjectival).
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