cxd5 is the correct notation.
d5 is a pawn push
xd5 I have never seen and for reasons it means very little.
The PGN notation I know (http://tim-mann.org/Standard) uses cxd5:
"18.104.22.168: Basic SAN move construction
"A basic SAN move is given by listing the moving piece letter (omitted
for pawns) followed by the destination square. Capture moves are
denoted by the lower case letter "x" immediately prior to the
destination square; pawn captures include the file letter of the
originating square of the capturing pawn immediately prior to the "x"
OBS.: SAN = Standard Algebraic Notation
Oddly enought, I have a modern book on chess called "Aprenda Xadrez
com Garry Kasparov" (portuguese translation of "Learn chess with Garry
Kasparov") that has figurine notation inside, and the captures made by
pawns doesn't shows part of the information (doesn't have the "x" or
the number). So, your capture 'cxd5' would be noted as 'cd'. Of course
it's not PGN, it's a book with some games in abbreviated
algebraic-figurine notation, and PGN uses SAN.
Cesar A. K. Grossmann
No, but the document linked (http://tim-mann.org/Standard)
is the official standard.