I can give you a simple answer. Kosovo had always been a multi-ethnic area with a substantial Albanian population. It was most probably majority Slavic up until the 16th and 17th century, when the territory was depopulated in large migrations of Serbs moving north. The largest migration took place in the 17th century, which left the fertile valleys and highland plains of Kosovo depopulated. Much of the Slavic Christian population sided with the Austrian occupation forces during the Austro-Turkish war of the late 17th century, and as a consequence moved northwards towards Habsburg territories. Much of this movement was also provoked by reprisal attacks of Ottoman irregulars on the Christian population. The fertile lands were settled largely by Albanian clans moving down from the mountains eastward. These were both Muslim and Christian Albanian groups, and most probably the majority of Albanians in Kosovo became Muslim only later.

Likewise, after 1878, when Serbia was awarded the towns of Nis, Leskovac and Vranje, the Albanian population, which made up to 30-40% of those towns, was expelled from them, mostly to Kosovo.

As far as I understand, most authors who have done demographic studies would agree that Kosovo was majority Albanian from the late 17th century onwards, and that the religious profile gradually moved towards Islam. Until the Serbian occupation in 1912, and to a large extent even after, Serb-Albanian relations were generally peaceful. Albanian Muslims visited Serbian Orthodox churches to pray, and vice-versa. There was a high degree of religious syncretism (simultaneous circumcision and baptism for children, for example) as well. Conflicts were generally socio-economic and not ethnic-religious until the age of nationalism in the late 19th-early 20th century.

Sources:

Miranda Vickers Between Serb and Albanian: A History of Kosovo

Richard Jensen Albanians and Serbs in Kosovo: An Abbreviated History