Tamil
Since time immemorial, there have been frequent migrations to Kerala from the neighbouring state of Tamil Nadu. The history of ancient Kerala itself is deeply intertwined with Tamil Nadu's ancient past, and the Tamil and Malayalam languages are closely related. The dialect of Tamil spoken today in the district of Palakkad in Kerala has a large number of Malayalam loanwords, has been influenced by Malayalam syntax and also has a distinct Malayalam accent.
Some of the earliest migrations attested by history were those of Iyers from the Cauvery delta to the district of Palakkad. The first of these migrations are believed to have taken place over five hundred years ago. These Iyers settled in Palakkad where they owned land and led an affluent existence till the enactment of the land reforms in the 1960s. They followed many Malayali practices include the practice of sambandham. There have also been migrations of Iyers to the princely state of Travancore from the Tirunelveli district during the 18th and 19th centuries. These Iyers are called Kerala Iyers and differ significantly from Palakkad Iyers in their language and social status. Some of Travancore's diwans were Tamil Brahmins. See: http://en.wikipedia.org/wiki/Travancore
Apart from Iyers, many other Tamil communities also migrated to Kerala between the 18th and the 20th century when the southernmost Kanyakumari district of Tamil Nadu was a part of the Travancore kingdom. The capital of the Travancore kingdom which ruled over most of Kerala was located in Padmanabhapuram in present-day Tamil Nadu. The caste system in Kerala is very similar to that which exists in the southern districts of Tamil Nadu.
Vellalars migrated from Tamil Nadu to eastern parts of Kerala like Idukki district, Pathanamthitta District. They are prominently found in Erattupetta, Pala, Ponkunnam, Kanjirappalli, Thodupuzha, Pathanamthitta etc. Similarly, Tamil Muslims migrated the same route from Madurai to Thodupuzha, Irattupetta, Pathanamthitta regions. They are distinct from Malayalam speaking Mappila Muslims in many ways including the usage of a dialect of Tamil inside their houses.
Read more about this topic: Ethnic Groups In Kerala