well, if both the domain are yours you just have to redirect the XYZ.COM to ABC.COM.
So if someone types xyz.com he will be forced to go to abc.com, you can do that with a code or sometimes by your domain control panel.
CODE
<META http-equiv="refresh" content="1;URL=http://abc.com">
You can even set the time, you can aslo use Javascript but if they disabled it... they wont be redirected, i think it can be done via htaccess too but i dunno how.
Than there is an other technique using an iframe and that is if you do not own the website.
You add an iframe :
CODE
<iframe src="http:www.abc.com" width="100%" height="100%" mane="what ever"></iframe>
So once you are on the xyz.com site it will set an iframe with the abc.com content in it. Problem is no urls will work your url will always stay xyz.com. Plus if you want to iframe a site... probably means that you do not own it so the author would mind i think !