Sample Package.xml:

<?xml version="1.0" encoding="UTF-8"?>
<Package xmlns="http://soap.sforce.com/2006/04/metadata"> 
 <types>
  <members>Object Name.Validation Name</members>
        <name>ValidationRule</name>
    </types>   
 <version>30.0</version>
</Package>  

Note:

Use API names for Object Name and Validation Name.