sorry, some explanation.
From your post.
Did you want access to the raw SAML response XML?
If so, may I ask why you need this access?
The SAML response XML is available through the SAML low-level API but it isn't exposed in the high-level API (SAMLServiceProvider.ReceiveSSO etc). Normally you don't require the details of the raw SAML response.
If so, may I ask why you need this access? - I need it to validate the supplied XML against the SAML XML Schemas after SAMLServiceProvider.ReceiveSSO called, or before. Doesn't matter in fact.
So, my goal is to validate the assertion and if it's not validated, deny login. How can i implement it without low-level API?