Confirm/Success: 

ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.Confirm,’Sample Success Message’));

Warning:

ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.Warning,’Sample Warning Message’));

 Informational:

ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.Info,’Sample Informational Message’));

Error:

ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.Error,’Sample Error Message’));