Schedule Migrate to import XML data regularly
I think the best way to achieve your goal is to use Drupal's cron, not Rules.

Cron is a service that will trigger the execution of a function, depending on the frequency you want it to.

You just have to implement hook_cron in your custom module to execute your migration. You will have to take care of the frequency you need for your script. I found this example useful.

How to run migration programmatically? As stated here, it's very simple:

&lt;?php
  $migration = Migration::getInstance($machine_name);
  $result = $migration-&gt;processImport();
?&gt;