The Collaborative International Dictionary of English v.0.48:

kvass \kvass\, n. [Russ. kvas'.]
   A thin, sour beer, made by pouring warm water on rye or
   barley meal and letting it ferment, -- much used by the
   Russians. [written also quas and quass.]
   [1913 Webster + PJC]

The Collaborative International Dictionary of English v.0.48:

Quass \Quass\, n. [Russ. kvas'.]
   A thin, sour beer, made by pouring warm water on rye or
   barley meal and letting it ferment, -- much used by the
   Russians. Called also kvass. [written also kvass and
   quas.]
   [1913 Webster + PJC]

WordNet (r) 3.0 (2006):

kvass
    n 1: fermented beverage resembling beer but made from rye or
         barley