Use Sed to Replace a Specific String with Slashes

The slashes need to be escaped:

sed -i 's|serv\\b\\Param\\||g' ~/Desktop/3.reg

In sed, a single backslash is usually a escape character for something. For example, in GNU sed, an escape-b, as in \b, is interpreted to mean a word boundary. To prevent such interpretation, place two backslashes in a row where ever you want to match a single literal backslash.

Example

Based on your sample (updated as per the comments), let's start with this file:

$ cat 3.reg
serv\b\Param\
abc serv\b\Param\ def

Applying the above sed command:

$ sed -i 's|serv\\b\\Param\\||g' 3.reg
$ cat 3.reg

abc  def

The pattern is successfully removed.

The basic problem is that \ introduces various sequences in regular expressions, so to match \ in the input stream you need to use \\ in the RE.

It is not clear whether the quotes are part of the string you want to remove: from your fourth example I would assume not, in which case this example works if you double the back-slashes:

sed -i 's:serv\\b\\Param::g' ~/Desktop/3.reg

You do not need double-quotes in the search or replacement strings unless they form part of the search criteria: the single-quotes around the edit expression already remove any special meanings as far as the shell is concerned, so sed sees the literal expression.