All Scheme implementations display the value of the last evaluated expression before the program terminates.
"Hello world!"

The <tt>display</tt> and <tt>newline</tt> procedures are found in specific modules of the standard library in R6RS and R7RS.
The previous standards have no concept of modules and the entirety of the standard library is loaded by default.

===R5RS===
(display "Hello world!")
(newline)

===R6RS===
(import (rnrs base (6))
        (rnrs io simple (6)))
(display "Hello world!")
(newline)

===R7RS===
(import (scheme base)
        (scheme write))
(display "Hello world!")
(newline)