Her accused attacker, 25-year-old Haiyang Zhu of Ningbo, China, knew the victim but no motive for the slaying has been determined, Flinchum said. School records showed that Haiyang was listed as one of Xin's emergency contacts.
Haiyang was charged with first-degree murder and was being held without bond at the Montgomery County Jail. It was not immediately known if he had an attorney.
Haiyang and Xin had been having coffee in a cafe in the Graduate Life Center, where Xin was living. About seven other people who were in the coffee shop told police that the two hadn't been arguing before the attack.
Police received two 911 calls shortly after 7 p.m. Wednesday, Flinchum said, and were on the scene in a little more than a minute to take Haiyang into custody.
Flinchum said Haiyang was not known to the campus police or to the university team that deals with mentally disturbed students.
University officials said Haiyang arrived on campus last fall and was a Ph.D. student in agricultural and applied economics.
The stabbing was the first killing on campus since a mass killing on campus in 2007, when a student gunman shot 32 people and then took his own life.
"An act of violence like this brings back memories of April 16," university President Charles Steger said. "I have no doubt that many of us feel especially distraught."
University spokesman Larry Hincker said a campus alert system put in place after the mass shootings by Seung-Hui Cho in 2007 sent out messages to 30,000 subscribers by e-mail, text messages and telephone voice mails Wednesday night.
Because a suspect was in custody, the messages were sent out as notifications rather than as emergency alerts, he said.
Virginia Gov. Timothy M. Kaine offered condolences to the campus.
"My thoughts and prayers are with the friends and family of Xin Yang today — and with the broader Virginia Tech community," he said in a statement. "The tragic attack on campus this week has no doubt revived terrible memories for countless members of the Hokie family."