As I understand it, you have some inquiries on the support IO standard for CV XCVR refclk. For your information, you can choose either of 1.2 V PCML, 1.5 V PCML, 2.5 V PCML, Differential LVPECL, and LVDS IO standards. You may refer to the CV device datasheet -> "Reference Clock Specifications for GX, GT, SX, and ST Devices" section for more details.
Note that you can use HCSL only for PCIe application.
Please let me know if there is any concern. Thank you.