Is it OK to contact conference chairs and volunteer to review (specific to Data Science Conferences)? I am a final year Ph.D. student. I work in the area of Applied Data Mining and Visualization. I have published at both KDD and IEEE VIS. I primarily work in the area of Natural Language Processing, Social Media Analysis, Information Extraction and Data Visualization. 

However, I have still not received any opportunity to review papers. 

As per the suggestions on other questions on this forum (How to become a reviewer for an engineering journal?, How do you earn opportunites to review journals or conference papers?, How to show interest to serve as a TPC or reviewer in top conferences?, etc) there are two ways I could get an opportunity to review papers:


Publish papers (I have published three papers at KDD, 1 as first author and 2 as co-author but have not yet received an invitation)
Contact the Chairs and volunteer to be a reviewer 


Question: Since I have still not received an invitation to review, is it OK to contact the chair or PC and volunteer to be a reviewer? Do you as a chair, frown upon such emails?  

Note: This question is specifically for people who has served as PC / Chair for one of the top Data Science and/or Data Visualization conferences (or workshop) like KDD, ICML, ICDM, PKDD, CIKM, WSDM, IEEE VIS, CHI, etc.
There seems to be an explicit question, why you have not been invited to review papers, and an implicit one, why you have not been invited as a PC chair.

Reviewing papers

As a PhD student you should have been given papers to review, but these papers will not be given to you by the PC chair but rather PC members who may delegate one or two papers to you. The most likely people to delegate paper reviews are your supervisor(s), you could ask them.

Becoming a PC member

Since you mentioned volunteering to be a reviewer, it seems you might have being a PC member more in mind. Typically, you can only be a PC member after obtaining your PhD. At this point, people who know you should suggest your name, such as your current supervisor(s), people you meet at a conference (this has happened with me) or your new colleagues (e.g., a PI) if you move on after your PhD.

For the specific question, if you know the general chair quite well, then I think it is okay to ask if you can help out as a PC (once you have your PhD). If your supervisors or other close collaborators are SPC, you can also ask them to suggest your name. But I suspect once you have your PhD you will be asked anyway. 

This is only based on personal experience, no-one can say for sure whether a given person will mind.