\documentclass{article}

\usepackage{fontspec}
\usepackage{xltxtra}
\usepackage{xunicode}

\newfontfamily{\SAV}[Mapping=velthuis-sanskrit]{Devanagari MT}
\newfontfamily{\SBV}[Mapping=velthuis-sanskrit,Script=Devanagari]%
  {Devanagari MT}
\newfontfamily{\SCV}[Mapping=velthuis-sanskrit]%
  {Sanskrit 2003}
\newfontfamily{\SDV}[Mapping=velthuis-sanskrit,Script=Devanagari]%
  {Sanskrit 2003}

\newfontfamily{\SAU}{Devanagari MT}
\newfontfamily{\SBU}[Script=Devanagari]{Devanagari MT}
\newfontfamily{\SCU}{Sanskrit 2003}
\newfontfamily{\SDU}[Script=Devanagari]{Sanskrit 2003}


\begin{document}

A: {\SAV .h .m atrya}

B: {\SBV .h .m atrya}

C: {\SCV .h .m atrya}

D: {\SDV .h .m atrya}

Unicode:

A: {\SAU ः ं अत्र्य}

B: {\SBU ः ं अत्र्य}

C: {\SCU ः ं अत्र्य}

D: {\SDU ः ं अत्र्य}

\end{document}