I'm guessing you are referring to the Swedish Democrats ( 2,93% in the last election) as the extremists, but I really don't think (against the popular opinion) it's a an accurate description of them. Yes they are nationalists, very conservative, have a twisted idea of how an older Sweden was, picturing some kind of peaceful core-family where everyone is happy in the country side.

They strongly believe that Sweden is loosing its cultural identity and don't agree with the notion of a multi-cultural country, thinks immigration in general is a bad idea and when it happens it should aim for cultural assimilation (language tests, living here for 10 years etc..), these are all things you can have a serious discussion about. Maybe it's true that we are loosing our national identity, and there are points to be made why that might be a bad thing. None of that is undemocratic (as opposed to fringe revolutionist communist groups), they aren't blatantly racist (as opposed to the National Democrats with 0,06% in the last election that has a program about for a white ethnic Sweden and the destruction of non-Christian religious buildings), they aren't violent (as opposed to some fringe Anarcho-syndicalist groups) or saying we should throw out all the immigrants out of the country, but since we have no debate about those issues and have a national implicit policy of not engaging the Swedish Democrats they just seem that way. There are a fair many countries that have nationalists and real conservatives in government like that. In fact in a fair many countries it's not weird to be proud of your culture and openly express it.

Now I happen to disagree with every single thing they stand for even though I agree that we are becoming a more heterogeneous society, and my own views is that we should increase globalization, increase immigration, give more power to the EU and just generally disagree with the notion of the type of cultural conservatism that conservatives speak isn't an achievable goal. These are all views you can engage and make points for and against.

Second to them in size there's the pirate party (0,63% in the last election), which I don't think anyone really thinks its necessary to not give seats, since they aren't more extreme than the left party (reformist communists), the green party or the more free-market bent Moderates (European type liberal-conservatives) in any event. Then the Christian Democrats might not make the 4% after the next election loosing all their seats, and while I strongly disagree with the very foundation of their platform, I certainly don't think they are extremists (we don't exactly have overzealous religious people in this country).

So I don't at all understand who our so called extremists are and what makes them so dangerous that they will taint the majority rule of parliament to some extreme degree. We've had a reformist communist party allied with our usually-ruling Social Democrats for as long as I can remember. They've been in parliament itself since before WWII and yet I don't think we have overreaching socialism compared to other Scandinavian countries or say Canada.

People relate to the minority parties as they do to homeopathy, like they'll have some absurd position of power if they go in as a function of dilution. None of our current parties broadly support the policies of the Swedish Democrats, even if there's some overlap between them and the Peoples Party (European type liberals) and the Christian Democrats.

In this years election it also looks like the Swedish Democrats are going to go over 4%, what are we going to do then.. raise the bar? All this boils down to the majority censoring the minority. The worst part of it is that voting on a minority party is almost equivalent to throwing your vote away, since we don't roll to votes we deem as to small to count over onto peoples second choice so some people simply end up having no representation by design.

In the Netherlands they have proper proportional voting which is considered a good thing.

TL;DR We have no extremists that are even close to getting into parliament, only minority parties that are being branded extremists because they have unpopular but democratic platforms. Democratic in the sense that they are no worse than the current parliamentary parties.