I am sending payment to user of my application but want to validate that Is Paypal account is exist on this email or not?
The only way to know if there is a PayPal account attached to an email address prior to a payment is to use Connect with PayPal to have the user log in with their account, and validate the response.
Otherwise, if you send a payment to an email address that has no account, they will receive a notification that there is money waiting and it will be auto-refunded after 30 days (with a reminder) if they do not claim it by creating an account or adding that email to an existing account.