The origin of the name the Kievan Rus' has a lot of debates, especially in some countries with strong influence on nationalist ideas(for more information check ethno-nationalist foundation stories). There was once controversy over whether the Rus' were Varangians or Slavs.  Some historians argue that the Rus' were themselves Slavs, while Varangians were Norse traders and Vikings. The close connection between the Rus' and the Norse is confirmed both by extensive Scandinavian settlements that were found on nowadays territories of  Belarus, Russia, and Ukraine and by Slavic influences in the Swedish language.

Putting aside the pro-nationalist politico-historical view, we could follow historical and archeological findings that explain tribes' formations on the territory that was later names, Kievan Rus'.  

Liutprand of Cremona, identifies the "Russi" with the Norse but explains the name as a Greek term referring to their physical traits. Leo the Deacon, a 10th-century Byzantine historian and chronicler, refers to the Rus' as "Scythians" and notes that they tended to adopt Greek rituals and customs. But 'Scythians' in Greek parlance is used predominantly as a generic term for nomads.

Before the Kievan Rus' creation or appearance, most of the area north of the Black Sea was primarily populated by eastern Slavic tribes. But it also was surrounded and populated by many more as Ilmen Slavs and neighboring Krivich, Finnic Chud tribe,  Poliane, a group of Slavicized tribes with Iranian origins, the Drevliane and the Severiane together with the Vyatichi, and to their south was forested land settled by Slav farmers, giving way to steppe lands populated by nomadic herdsmen.