Is there any restriction on employment of foreign nationals

Employment of foreign nationals is not restricted by FEMA at all. The parameter term `employment' is broader than 'working'. Employment could be as technicians or non-executive directors or as full-time directors (called whole-time director or managing director). Technicians could be engaged for short terms by a contract. The exchange control law does not restrict the duration of such employment as long as the employee holds valid employment visa.
As for non-executive directors, the procedures prescribed under the Companies Act, 1956 alone need to be followed. These include filing forms for such appointment, consequent to the appointment by the board of directors or by the shareholders in general meeting. If the appointment is in a public limited company, the consent to act as director is to be obtained from the foreign national.
A private limited company is not subject to any restriction under the Companies Act on appointing foreign nationals in key managerial positions. Appointment of foreign nationals as whole-time director or managing director in a public limited company is s subject to the provisions of Section 269 of the Companies Act read with Schedule XIII to the Act that defines qualification for appointment and ceiling limits of remuneration.