Slovenes are Slavs who arrived in the Balkans around the 6th century. They escaped Avar domination and in the 7th century were free. In the 8th century the Franks under Charlemagne overran them and brought with them Christianity.
Croats are a South Slav branch of Indo-Europeans. Constantinople's emperor in the 610s and 620s is described as having invited the Croats down from Central Europe to counter the threat from Avars. The Croats brought some neighboring Serbs with them, and they settled on land abandoned by the Avars.
Singidunum was from the time of the Roman Empire, rebuilt by Justinian in 535, then invaded by Avars. Serb settlers arrived around the year 630.
Serbs are a South Slav branch of Indo-Europeans. Similar to other Slavs, they came to the Balkans from central and eastern Europe, perhaps from around the southern Volga River.
Main towns in this area retained their Latin speaking populations and remained under Constantinople's rule. Some Latin speakers would eventually become Slavicized. Latin-speakers survived the coming of Serbs in parts of the central and west-central Balkans. They were semi-nomadic shepards, horse-breeders and muleteers. They were called Vlachs, a word used by Serbs to describe people who spoke a Latinate language.