Passing a single parameter in XsltListViewWebPart:

<a href=http://<url>/<page>.aspx?<param1>={$thisNode/@param1value}>

Passing multiple parameter in XsltListViewWebPart:

<a href="http://<url>/<page>.aspx?<param1>={$thisNode/@param1value}&amp;<param2>={$thisNode/@param2value}">

Remember that "&" is not enough, you will need to put in "&amp;"