And that brings up a previous point Beni made. The combatants can't just go anywhere. (Unless Beni wants to clear that up a bit more)
Agreed, considering Xizor actually has his own personal Starfighter that has a Hyperdrive, and Guri has her own personal starship...
In all honesty, Xizor's chances primarily depend on how much of a threat Exar Kun and Traya would consider him to be.
I really think Exar Kun has some raw advantages in terms of numbers and fighting strength, but at the same time I think Sion could take down Ulic and Mandalore.
Both Xizor and Kun have Mandos so that would be a wash (assuming some mandos don't recognize that mandalore as legit)...
Really, Xizor's palace didn't have the capability to handle space battles and quite frankly it didn't need to, because that was the perview of Coruscant's planetary defenses...
As far as Traya's Academy, I don't see how Exar Kun could even damage it with his fleet, let alone destroy it, barring a super weapon. It survived Malachor V being ripped apart back at the end of the Mandalorian Wars, remember.
So Xizor's survival is entirely dependent on whether or not Traya and Exar Kun consider him to be a serious threat.
Exar Kun: Well in a typical Sith Lord fashion, Exar Kun would likely consider Xizor to be a laughable threat, Kun's flaw is his overconfidence, so he may go gunning for Traya first, thinking he can eliminate Xizor at his leisure. Unlike Revan, he lacks the patience to wait out his opponent and let his opponent make the mistake of launching a stupid attack, so it stands to reason that Exar Kun would try to invade Traya's Academy.
Traya: She would consider Xixor to be a key player in this, however if you look at her behavior patterns, she's more likely to go gunning for Exar Kun first (considering her hatred of the Force), and Kun isn't giving her proper respect as a Dark Lord of the Sith... While her patience isn't infinite, she has more patience than Exar Kun and thus she'd lure Exar Kun to where she has the home court advantage, Ravenger and the other hulks at her disposal wouldn't likely be enough to take down Exar Kun's forces.
Xizor: He's going to make sure he has good defenses, and isn't likely to act before he is ready. While he's shown overconfidence in the past, he isn't as likely to make that mistake like he did concerning Luke Skywalker, with the forces that both Traya and Kun command, it's plainly obvious that he needs to take them very seriously. He also isn't going to whimsically dispatch Guri to eliminate people, it's a waste of time and likely going to send Guri to the scrap heap.
What I see is Exar Kun blockading Malachor V and losing patience and in his overconfidence, he launches an all out invasion of the planet. As his troops get eliminated by various examples of the local wildlife, they then end up having to take on a bunch of sith assassins, troopers, Dark Jedi, other force wielding sith, etc. This would likely whittle down the numbers on both sides.
I see Sion easily dispatching Ulic and Mandalore... I'm just not sure if Exar Kun will have the presense of mind to try to talk Sion into giving up. Remember, the Exile didn't Force Persuade Sion to give up, the Exile persuaded Sion purely with words. I really don't know who'd win this fight, while Exar Kun is the better duelist, the fact Sion just won't die, means that he likely would be able to outlast Kun. So it's hard to say.
If Kun wins then he has to take on Traya, Kun is the better saber wielder, but I think Traya might be the better force wielder. Also she has some stealth abilities that I'm not sure Exar Kun has or at least not as developed as Traya's were.
I kinda think Traya will defeat Kun in this purely due to Kun's impatience causes him to confront Traya on her own turf.
Now depending on whether or not Xizor's intelligence has figured out where this huge confrontation has taken place, he'd probably have forces show up while Exar Kun's forces are on Malachor V, and wait for the victor to emerge.
Now if he manages to successfully keep the victor's forces from leaving the planet and then mine the surrounding space, he may actually prove the victor.
If he fails at this, it turns into a possible confrontation on Xizor's home ground, but at that point I believe Xixor could win (barring thermal detonators aren't in the mix). Xizor would probably opt for Sion to be vaporized since not even immortal sith could come back from being reduced to particulate matter (or send Sion's rebuilding body into the heart of the Coruscant Sun).
Xizor would probably win this if the Sith decide to go kill each other first, if Exar Kun decides to go hunting for Xizor first though, Xizor is in serious trouble.
I consider the most likely winners to be either Traya or Xizor, it all boils down to which individual Exar Kun decides to attack first.
While Exar Kun had a Sith Amulet, I'm not sure it would be effective on Sith Assassins that get more powerful, the stronger their adversary is, especially on Traya's home turf. I also don't think he could force persuade Sion into committing suicide out of hand...