How to retrieve all custom objects from Ant Migration in Salesforce?

Hi,

For Standard Objects :-

<?xml version="1.0" encoding="UTF-8"?>
<Package xmlns="http://soap.sforce.com/2006/04/metadata">
<types>
<members>Account</members>
<name>CustomObject</name>
</types>
<version>40.0</version>
</Package>

For Custom Objects :-

<?xml version="1.0" encoding="UTF-8"?>
<Package xmlns="http://soap.sforce.com/2006/04/metadata">
<types>
<members>*</members>
<name>CustomObject</name>
</types>
<version>40.0</version>
</Package>

Hope this helps.