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.