Incomplete (abandoned) event registrations
CiviCRM doesn't store data that has been partially submitted. If your form uses paypal standard payment method and user fills in the other details and pay's now then he will be taken to paypal screen where he will need to enter card details or pay using paypal account. Before going to Paypal CiviCRM creates Pending Participant records and then changes status to completed if it receives payment.

So you will need to either have registrants submit 3 step form or build extension that would do one of the following


Don't use confirmation page incase for Pay later and/or Pay now.
Store Data in CiviCRM when user submits the main form, Truncate if user submits confirmation, Add report to list all the user that submitted partially.


(#1 will be simple and #2 will need more work)

HTH

Pradeep