How easy is it to invite top researchers to come and give a talk and interact with students/faculty in India? Are there allowances for travel reimbursement for such scientists?
Very much possible, but obviously, budgets do not stretch to inviting a foreign researcher to just visit for a day and give a seminar (you can use your own travel funds to invite a visitor, but foreign travel funds are pretty constrained, as has been described in a different section). Usually, overseas scientists are invited as part of a topic-specific meeting organized by an Institution or an across-Institution group of scientists. Often, there may be student-focused training workshops or lecture series associated with the meeting- this component is what granting agencies will provide travel funds for. For example, the IBRO offers funds for workshops in neuroscience for which scientists from Asia-Pacific countries can be invited. The Frontiers of Science meetings (Indo-UK, Indo US) are another series that pay for travel of scientists from the respective countries to India.
Sometimes, PIs do some networking and take advantage of the planned visit of an overseas scientist to one Institution, to plan a local "travel junket" so that seminars can be arranged at a couple of more institutions. We find top researchers who are willing to fly out to India for a meeting are usually willing to go to the trouble of traveling to one or two other places and give seminars there too. What it takes is good networking on part of the Indian PIs- a network we hope to nurture and strengthen among YIs, with efforts such as the YIMs.