Yes and no. They're definitely "An authoritarian and nationalistic right-wing system of government and social organization", but then again, so are the Republicans and Democrats. Some people claim Mussolini defined fascism as corporatism, but I've never seen anyone cite a real source for that quote. He did however say, the fascist doctrine was "All within the state, nothing outside the state, nothing against the state", which I doubt many KKK supporters would have endorsed in previous eras, though their affinity for Protestant Constitutionalism could easily become fascist theocracy during crisis.
The 1st wave KKK (1860s-70s) reacted to the Reconstruction Era and undertook paramilitary, terrorist acts against freed slaves and their supporters. They had a more rural basis. One could make the case that they resembled the fascist Brownshirts and Blackshirts, but they did not act to solidify the power of a political party, much less a revolutionary one.
The 2nd wave KKK (1920s) reacted to demographic changes in urban immigration. It became an open fraternal organization with formal recruitment and policy. They used civic engagement with parades, insurance policies for members, and white Protestant women's groups. They continued racist assaults on blacks and Jews, assaulted IWW organizers, and tied in with political struggles such as Prohibition and anti-immigration policies especially, especially toward Catholic and Jewish immigrants. They had a strong urban presence. Between the 2nd and 3rd waves Hitler and Mussolini tied in with the Catholic Church, so there's definitely some overlap based on formal organization and anti-semitic, anti-black bigotry, but also some contrast due to regional power relations. The 2nd wave though had some strong resemblances to fascist Germany and fascist Italy with all of the formal organization and anti-immigrant rhetoric, emphasis on drama, parades, occultism, etc.
The 3rd wave KKK (1950s-60s) reacted to desegregation efforts, and the Civil Rights Movement generally. They escalated infiltration of political office. Developing from 40s anti-union attacks and bombings, they continued to lynch and bomb, especially toward "upwardly mobile blacks". By the beginning of the 50s they had bombed the homes of at least 40 black Southern families. The FBI used COINTELPRO to target civil rights activists and Left militants instead of the KKK during this period. We see increasing similarity to Nazis via white power salutes (resembling German seig heils, coming from the Roman Empire's arm salute, that Mussolini also adopted for Italian fascists). Yet a lot of American patriotic veterans had just fought fascists in World War II and so were not so enthusiastic about bringing European fascist ideology to the US. Those who did see it as favorable joined the American Nazi Party of 1960, otherwise known as the "World Union of Free Enterprise National Socialists" (free enterprise cuz this is 'Murica!), not the KKK.
Current KKK activity comes primarily through alliances with Neo-Nazis and other non-KKK white nationalists & supremacist groups, which are increasingly assimilating the KKK into their own style. A lot of other white nationalist groups, especially Neo-Nazi ones, look down upon the KKK as disorganized, uneducated, pillow-case wearing clowns, and have overshadowed the KKK in the US. It's hard to talk about the KKK as an independent entity anymore; although there are of course local chapters, they have not had as much influence on white supremacist culture or made many gains or achievements since losing the civil rights battle.
Fascism has primarily incorporated Third Position opportunism, of arguing for economics other than "socialism" and "capitalism". Fascists are, historically, revolutionaries, while conservatives are reactionaries. The KKK have never nominally had any economic critiques beyond the run-of-the-mill conservative anti-communist rhetoric, or revolutionary attitudes and aspirations. The KKK have no critique of capitalism beyond vulgar anti-Jewish banking sentiments, whereas fascists sought to ride the waves of growing anti-capitalist notions. If anything they want to return to the past, not create, for example, a 1000 Year Empire. They do have a militaristic, nationalist history, occultist tendencies, formal organizational capacity, infiltration of police and political office, and violent white supremacist direct action, which are compelling resemblances. They've had very similar targets. They have a lot of overlaps, but I would distinguish them; fascism inherited national syndicalist and monarchical traits, while the KKK inherited Antebellum South and Constitutionalist traits, and because of this, they fundamentally diverge. Of course, the KKK still remain white supremacist trash to fight at every chance, but the KKK do not necessarily follow the fascist pattern of totalitarian opportunism.
