Relax ng is an xml schema language like dtd or w3c xml schema. relax ng schemas can be written in either xml (.rng files) or an alternative compact syntax (.rnc files). rnv uses the latter to check if a give xml file is valid in respect to the language defined by the relax ng schema. rnv uses expat for xml parsing.

if you intend to validate against relax ng schemas in xml syntax instead have a look at xmllint (package libxml2-utils) or xmlstarlet. also trang can convert relax ng schemas from xml to compact syntax and back.