#1
|-x--x----x|
|-----------|
|1--------2|
|-----------|
|-----------|
|-----------|
what dose the "x" mean?
#3
x's never mean palm mute. x's always mean that the string is played, but it is muted. That is a strange tab since the e string is muted but not the B string. This leads me to believe that the lick is played with your fingers. I would play this tab by plucking the G string with my thumb and mute the e string while plucking it with my index or middle finger.

When a part of the song is palm muted it will be notated like this...

|-----------------|
|-----------------|
|-----------------|
|--5--5--7--7---|
|--5--5--7--7---|
|--3--3--5--5---|
.........|..pm..|

where the final two power chords are palm muted.