Sample package.xml to retrieve Picklist Value Set/Global Picklists

package.xml:

<?xml version="1.0" encoding="UTF-8"?>

<Package xmlns="http://soap.sforce.com/2006/04/metadata">

    <types>

        <members>*</members>

        <name>GlobalValueSet</name>

    </types>

    <version>38.0</version>

</Package>