XMRV refers to a recently discovered retrovirus called xenotropic murine leukemia virus-related virus. It was first identified in 2006 in samples from men with prostate cancer and later to chronic fatigue syndrome. There are only 3 known human retroviruses: HTLV, HIV (which leads to AIDS), and now XMRV. In 2009, researchers reported finding XMRV DNA in two thirds of 101 people with chronic fatigue syndrome (CFS) but in only 4% of healthy controls.  It has also been proposed to be the cause for conditions including autism, fibromyalgia, multiple sclerosis, amyotrophic lateral sclerosis, and Parkinson’s disease. Later research could not find a correlation. 
For more info please read
How XMRV Retrovirus Findings May Fit With The Amygdala Hyperarousal Model for ME/CFS
A primer for XMRV, XAND, M.E. – and CFS By Mary Schweitzer, Ph.D.
Interesting Note From Wiki:
Interesting fact- from the CDC site:
While it is presumed that XMRV and MLVs can be transmitted through blood transfusion, no such transmission has been identified, and there are no known cases of XMRV or MLV infection or related illness in transfusion recipients. Therefore, there is currently no requirement for testing of the blood supply for the presence of XMRV or MLVs.
2 Xenotropic Murine Leukemia Virus-Related Virus in Chronic Fatigue Syndrome and Prostate Cancer
3 No Evidence for XMRV in German CFS and MS Patients with Fatigue Despite the Ability of the Virus to Infect Human Blood Cells In Vitro
4 XMRV (Xenotropic Murine Leukemia Virus-related Virus) http://www.cdc.gov/xmrv/questions-answers.html#8