Many stoves are sold with the claimed ability to burn different fuels, but not all fuels are created equal.
White gasoline, kerosene, and canister gas are the best fuels for a petroleum type stove, period. You can get by sometimes with aviation gasoline, jet fuel, and automotive gasoline, but they should always be a second choice and they may shorten the life of the generator of the stove and will require more cleaning/maintenance. Diesel is never a good choice, no matter what the directions on a stove say. Fuel oil and home heating oil are equally bad. "Bunker" fuel (used for ships) should be avoided at all costs; it's even worse than diesel. Eat your supper cold before considering the use of bunker fuel.
Kerosene is typically the internationally available fuel. Bring some coffee filters and filter what fuel you buy through a coffee filter, and you'll avoid a lot of grief that way as you put it into your fuel bottle.
Fuel names vary by region. Kerosene isn't always called kerosene. In some parts of the English speaking world, it's called paraffin. Outside English speaking countries, the sky's the limit. The best, most current international fuel names list that I know if is maintained by Doron Papo and can be found at: International Fuel Names.
Specific Stove Recommmendations
OK, so above, you have my general thoughts about fuel types. Let's talk specific stoves.
As much as I like the Svea 123R, I don't think it's a good choice for international travel. It runs best on white gasoline which is not readily available world wide. Yes, you can run it on automotive gasoline, but the stove will clog more often. The Svea 123R is definitely a better choice than an Optimus 8R, Primus 71, Optimus 80, or Optimus 99. The Svea 123R is still in production; those other stoves in the same class are not. There's a reason for that. A Svea 123R wouldn't be a bad choice, but it cannot run on kerosene.
The MSR Dragonfly has an inline filter at the tip of the fuel line. This filter can clog easily on dirty fuels. Not a problem in 1st world countries, but I wouldn't take it to a 3rd world country. It's a more sensitive stove than some of the others.
The MSR XGK is a very robust stove and will burn just about anything. It doesn't as you point out simmer well. You can bring a simmer plate for it.
The MSR Whisperlite Universal will simmer. It takes some fiddling, but it will simmer, and it will simmer far better than something like a Soto Muka. You have to have low pressure in the fuel bottle (1/5th the normal number of pumps) and the bottle needs to be half empty. The jet is pretty easy to change on a Universal, and the Universal will burn canister gas, which is nice where available. Of course the Universal can only use threaded canisters. In many parts of the world, only puncture type canisters are available. In other parts of the world valved canisters will be available, but they'll be the non-threaded kind (Camping Gaz).
The Primus Omnifuel and Omnilite are very nice stoves. They have an advantage with canister gas in that they do not need a fitting change to burn canister gas vs. liquid fuel where as a Whisperlite Universal does. They are excellent simmering stoves. They're a little more complex than a Universal, and they will perhaps require a bit more maintenance. Either would be a good choice, the Omnilite is lighter and better for 1 to 3 persons. For more than 3, perhaps the Omnifuel is a good choice.
The Optimus Nova has had a lot of quality control problems as of late. Supposedly they are fixed now, but...
There's my take on the various stoves. A lot will depend on which countries you're going to and what cooking capabilities you want to have. An XGK might well be the most robust of the above lot, but it's not really a cook's stove. It's a water boiler and snow melter. It was originally designed by a mountaineer for mountaineering.
I definitely would not take a Dragonfly to a 3rd world country. Just too sensitive.
Maybe the Nova if they're really fixed the quality control problems.
That leaves the Universal or the Omnifuel/Omnilite. The Omnilite is the lightest of the bunch. If the price doesn't scare you, it's a nice stove. The Universal is cheaper and simpler. The Universal also has a cable down the fuel line. If deposits build up from using things like automotive gasoline, you can use the cable and a pair of pliers to scour out the generator and fuel line. The Omnifuel/Omnilite is a much better simmering stove (easy, can be done on a nearly full bottle, total control). The Universal can be made to simmer, but it takes some knowledge and some fiddling around.
There's my take. Hope that helps,
Adventures in Stoving