Вроде работает
<?php $result_par = mysql_query("SELECT MAX(data) FROM xolod WHERE id_kv='$id_kv'");
$myrow_par = mysql_fetch_array($result_par);
$prov = $myrow_par[0];
$result_par2 = mysql_query("SELECT MAX(data) FROM xolod WHERE id_kv='$id_kv' AND data<'$prov'");
$myrow_par2 = mysql_fetch_array($result_par2);
echo $myrow_par2[0];
?>