If you have imported your IPB 3.x database into XenForo, you can automatically redirect all traffic destined for your IPB content to its new location within XenForo.
All that is required is that you unzip these files and place them in the directory in which your IPB files resided. The included .htaccess file will take care of redirecting calls to IPB into to the ips2xf.php file, which will in turn redirect traffic to the appropriate traffic to XenForo.
Any requests of the old URLs will be redirected with a 301 permanent redirection header.
If you have installed XenForo in a different directory from where your IPB used to reside, you will need to edit the included 301config.php file in order to specify the full file path to your XenForo installation, as in this example:
$fileDir = '/home/example/public_html/xenforo-forum';
If your IPB import is (or will be) one of several imports into your XenForo, you will need to instruct the redirect scripts how to find the archived import log data by specifying the import log table name.
Full details are in the XenForo Manual: