You say that the listing is a custom post type so each job listing has a post ID so you can just pass that to your uploadcv.php in your Apply Now Button :
<a href="https://wordpress.stackexchange.com/questions/22457/.uploadcv.php?listing_id=<?php echo $post->ID; ?>">Apply Now</a>
and then retrieve that in your uploadcv.php :
$listing_ID = (int)$_GET['listing_id'];
Now as far as saving the CV in the database, that would be a bad id, when you can simply upload the cv to your server and just save the URL in the database.