Yav (Jav), Prav and Nav (Navi) are three worlds used in the literary forgery Book of Veles. References to them became popular among Ukrainians and Russian neopagans, who claim that all three combine into Triglav that governs the Universe.
Yav is the material world in which we are right now. Gods, however, are equally part of Yav. Yav is contained within Nav.
Nav (Navi) is the immaterial world, the world of the dead. Stars, which are the souls of the dead, as well as Svarga and Irij, are parts of Nav.
The folk etymology relates Yav with Polish jawa, meaning the reality (not a dream, e.g. jawa lub sen - reality or a dream), with [jav]lenie or yavlenie meaning "manifest", "real"
Prav with Slavic [prav]lenie or [prav]o, which means "law", "rule", one's right to something, and the right side and is cognate with Latin probus.
Nav: with naive etymology of the Russian наверно [nav]ernoe "probably", where in fact it is na + vernoe (faith).