From the le- stem of van + -vén (adverbial-participle suffix), compare lesz, lenni, and lét, lévő.
lévén
This participle is currently undergoing a conversion into a conjunction introducing a subordinate clause, due to the fact that there are sentences where both interpretations are possible, especially when this word introduces a clause, and it may be reanalyzed as a conjunction. For example:
Due to this latter use, the word lévén may become virtually synonymous with minthogy or mivel (“as, since”). In the original participial usage, lévén would typically remain at the end of the clause, as part of a complex predicate (such as …gabonatermelője lévén (“being the grain producers of…”)), but this usage may be perceived as literary or dated. If the verb van (“to be, to have”) is not part of the predicate of the reason clause, lévén cannot be used as a conjunction, unless as part of the phrase lévén, hogy, which formally creates a separate clause. With respect to its original meaning, lévén(, hogy) may not be justified when the reason already involves the verb van, e.g. the phrasing lévén, hogy nincs kenyere (“since s/he doesn’t have bread”) may be disputed, and either mivel/minthogy nincs kenyere or a more old-fashioned nem lévén kenyere may be preferable instead from a prescriptive point of view.