Notation
In guitar tablature, palm mutes are rendered with a "P.M." or "PM", and a dashed or dotted line for the duration of the phrase to be muted. If the pitches of the muted notes are discernible, the fret numbers are given accordingly, otherwise they are represented with an X in lieu of a tab number. (If an X appears in lieu of a tab number but there is no P.M. directive, this usually means to mute the string using the fretting hand, not the picking hand.)
P.M.------------| e |------------------| B |--8-------8-------| G |--7-------7-------| D |--6-------6-------| A |--7-------7-------| E |----0-0-0---0-0-0-|Read more about this topic: Palm Mute