(The following examples are from The Linux Kernel Module Programming Guide by Peter J. Saltzman, Michael Burian, and Ori Pomerantz.)

Here is their first kernel module code.

/*  
 *  hello-1.c - The simplest kernel module.
 */
    #include <linux/module.h>/* Needed by all modules */
    #include <linux/kernel.h>/* Needed for KERN_INFO */

int init_module(void)
{
        printk(KERN_INFO "Hello world 1.\n");

        /* 
         * A non 0 return means init_module failed; module can't be
        loaded. 
         */
        return 0;
}

void cleanup_module(void)
{
        printk(KERN_INFO "Goodbye world 1.\n");
}

What is KERN_INFO?

Where does printk output go?

Output goes to a log file:

 /var/log/syslog
   

and will also go to the "console" (not to an xterm window) provided the priority level is sufficiently high:

From /usr/src/[kernel version]/include/linux/printk.h

#define KERN_EMERG      "<0>"   /* system is unusable */
#define KERN_ALERT      "<1>"   /* action must be taken immediately    */
#define KERN_CRIT       "<2>"   /* critical conditions    */
#define KERN_ERR        "<3>"   /* error conditions    */
#define KERN_WARNING    "<4>"   /* warning conditions    */
#define KERN_NOTICE     "<5>"   /* normal but significant condition    */
#define KERN_INFO       "<6>"   /* informational    */
#define KERN_DEBUG      "<7>"   /* debug-level messages    */

Changing KERN_INFO to KERN_ALERT will cause the output to go to the console.