Seems like you have full error reporting enabled. The error is actually a notice which you can turn off by changing the "error_reporting" value inside php.ini or inside the code.
Anyway, the problem is there and you should check for the presence of the key inside an array before attempting to access it. Lots of ways to do this. Here is the simplest one of them all:
$cvid = @$_GET['cvid'];
^---- error suppression operator
Alternate way is to use is_set():
$cvid = isset($_GET['cvid']) ? $_GET['cvid'] : '';
Yet another way is to use array_key_exists():
$cvid = array_key_exists('cvid', $_GET) ? $_GET['cvid'] : '';