I'm not sure about the tv_grab_url script but I can tell you what I do that has worked for some time now. I forget where I found it so sorry I can't reference the source.
I have "External: XMLTV" enabled under 'Configuration > Channels / EPG > EPG Grabber Modules'. If you select that option then on the right side it will give you the full path to the socket that it will accept direct input on. In my case that is /var/lib/tvheadend/config/epggrab/xmltv.sock.
I have a crontab entry that daily pulls the xml file I want and then pipes it to this socket. The last command of that is this: cat /var/lib/tvheadend/xmltv.xml | socat - UNIX-CONNECT:/var/lib/tvheadend/config/epggrab/xmltv.sock.
So, if you verify your xmltv.sock location in the path, and then pull one of those xml files, you should be able to use the command above to send it to the socket and then check tvheadend to see if the information gets populated. It looks like I'm running a ~ 2 year old version of 4.3 (Build: 4.3-2009~g1295dd2) so some things might have been updated since then.
