The normal way to send events to a SIEM server is via Syslog. I think all decent SIEM systems will be able to receive Syslog messages. The documentation describes how to set up a Remote Syslog Server Configuration. In general, the SIEM system would need to write/implement the parsers
Also, there is a specific PAM integration for Splunk as PAM includes a Splunk Forwarder and can send the data as key/value pairs. Splunk Configuration is also in the documentation.