The DM decides
Full quote from PHB-2014:
The DM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result.
There are a number of unusual circumstances in OP's situation, requiring some DM rulings on whether they would grant advantage to the Xorns' attacks.
Earthglide is weird
Xorn have a rather odd movement ability:
Earth Glide. The xorn can burrow through nonmagical, unworked earth and stone. While doing so, the xorn doesn't disturb the material it moves through.
Typically, a creature with a burrow speed "can use that speed to move through sand, earth, mud, or ice. A monster can't burrow through solid rock unless it has a special trait that allows it to do so." (MM8) The Earthglide of a Xorn is unusual both in that it permits them to move through stone and that rather than specifying the size of the tunnel created behind it, it explicitly "doesn't disturb" the material that it moves through. So what does that actually look like, narratively, for your DM? How are they describing that ability? Does the rock temporarily wink out of existence as the Xorn touches it, but reappear behind it? Are the Xorn and the rock somehow 'out of phase' with one another so that they both exist in the same place at the same time but don't physically interact? Does the Xorn walk around with a mobile passwall effect that creates a fully empty 5 x 5 x 5 cube around them? Details like these circumstances will affect how likely a DM is to give the Xorn advantage on their attacks. Since OP assumes the Xorn are unseen while in the stony walls of the cave, let's say it's not the last option, but one of the first two.
They are mutually unseen
If the Xorn are behind rock walls so that their targets cannot see them, neither can the Xorn see their targets. As DaleM points out in his answer, tremorsense does not grant sight. Rather (emphasis mine),
A creature with Tremorsense can pinpoint the location of creatures and moving objects within a specific range, provided that the creature with Tremorsense and anything it is detecting are both in contact with the same surface (such as the ground, a wall, or a ceiling) or the same liquid.
Tremorsense can’t detect creatures or objects in the air, and it doesn’t count as a form of sight.
So while the targets cannot see the Xorn, neither can the Xorn see them. If the Xorn leave the stone walls of the cave to use their darkvision, their target can then see them. "Unseen" will not be a source of advantage here. Also, as a note for any Dwarf with pet Xorn, you cannot 'call for them to attack'. Nothing in the description of the Xorn indicates that they can hear through solid rock either. I suggest you stamp your foot, a signal to attack that they can detect with their tremorsense.
They could be behind cover
However, rather than having the Xorn "emerge from the ground" to attack the party, why not have them stay in the stony walls, or floor, of the cave? If they cannot reach their targets from behind the walls without emerging, have them earthglide under their targets and attack from below.
RAW (assuming combat on a grid), for a Xorn that is beneath a character, the space it occupies is solid stone, while the space its target occupies is not. If these two spaces share a border, and assuming a five foot reach, the Xorn and the target should be able to attack one another. But does the solid stone space give either of them cover?
A target has total cover if it is completely concealed by an obstacle.
Unfortunately, the rules do not tell us what concealment actually means, despite JC tweeting about it as if we should know. If it meant unseen - there is already a word for that. And both parties are unseen - but the Xorn's tremorsense allows it to know the exact location of its unseen target, an advantage the target does not have. Further, to what extent is the solid rock an obstacle? It certainly is for the target, but if the Xorn's earthglide permits it to move freely through the rock at no loss of movement, is it an obstacle? A DM will need to interpret what the rules for total cover mean, decide whether a Xorn can have total cover from its target and still attack, and if so, whether that is a source of advantage.
They could be Hidden
If the DM does not want to implement Xorn attacking from below, the Dwarf might consider having the Xorn Hide before they attack. They can Hide while their targets can't see them, and they further get "advantage on Dexterity (Stealth) checks made to hide in rocky terrain" because of their stone camouflage. They might then emerge from the walls while Hidden and attack. Some DMs rule that if you are successfully Hidden, your first attack as you come out of Hiding has advantage. Others rule that as soon as you can see your target, your target can see you, and thus you are no longer Hidden (and see DaleM's thorough summary of the rules involved here). A lot depends on how the DM interprets Hiding (emphasis mine):
In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach a creature, it usually sees you. However, under certain circumstances, the Dungeon Master might allow you to stay hidden as you approach a creature that is distracted, allowing you to gain advantage on an attack before you are seen.
Is emerging from a rock wall using earthglide when the targets think that they are fighting a lone dwarf one such "certain circumstance"? Again, that is explicitly up to the DM to decide.
