I’m an international student who are applying to mostly LACs.
So on-campus interview is not an option for me… 
I realized that some colleges provide both alumni interview and a skype interview.
What is the different between the two?
I prefer meeting people on face to face, but I also feel talking directly with an admission officier will give a better impression on my application.
Please give me some advice!
In an alumni interview, you will meet with a graduate of that college, in person. Skype is obviously online. Any interview is better than no interview, in my opinion.
The skype interview is probably still an alumni interview, just an alumni interview done over skype rather than in a starbucks or whatever. I’ve done them (technically I used facetime) for domestic students who live in areas that don’t have enough (or potentially any) alumni to do in person interviews of everyone applying from there. Definitely better to do skype interview vs. no interview.