HVM vs PVHVM on CentOS 7 guest

[Pavel Hladik]

Hi, I'm bit confused, XenCenter shows that CentOS 7 virtual machines are (HVM). When I look into the CentOS 7 quest I see there is no modified kernel with `uname -a`, but `lsmod | grep xen` shows:

xenfs                  12667  1
xen_privcmd            13206  1 xenfs
xen_netfront           27206  0
xen_blkfront           26966  3

For me it looks like a PVHVM quest. I also see in `dmesg | grep -i xen`:

[    0.000000] DMI: Xen HVM domU, BIOS 4.13 12/16/2020
[    0.000000] Hypervisor detected: Xen HVM
[    0.000000] Xen version 4.13.
[    0.000000] Xen Platform PCI: I/O protocol version 1
[    0.000000] Netfront and the Xen platform PCI driver have been compiled for this kernel: unplug emulated NICs.
[    0.000000] Blkfront and the Xen platform PCI driver have been compiled for this kernel: unplug emulated disks.
[    0.000000] ACPI: RSDP 00000000000ea020 00024 (v02    Xen)
[    0.000000] ACPI: XSDT 00000000fc00dc20 00044 (v01    Xen      HVM 00000000 HVML 00000000)
[    0.000000] ACPI: FACP 00000000fc00d5d0 000F4 (v04    Xen      HVM 00000000 HVML 00000000)
[    0.000000] ACPI: DSDT 00000000fc0042a0 092A3 (v02    Xen      HVM 00000000 INTL 20160527)
[    0.000000] ACPI: APIC 00000000fc00d6d0 00460 (v02    Xen      HVM 00000000 HVML 00000000)
[    0.000000] ACPI: HPET 00000000fc00dbb0 00038 (v01    Xen      HVM 00000000 HVML 00000000)
[    0.000000] ACPI: WAET 00000000fc00dbf0 00028 (v01    Xen      HVM 00000000 HVML 00000000)
[    0.000000] Booting paravirtualized kernel on Xen HVM
[    0.000000] xen:events: Xen HVM callback vector for event delivery is enabled
[    0.126187] Xen: using vcpuop timer interface
[    0.126194] installing Xen timer for CPU 0
[    0.128434] installing Xen timer for CPU 1
[    0.255052] xen:balloon: Initialising balloon driver
[    0.268099] Switched to clocksource xen
[    0.274783] xen: --> pirq=16 -> irq=8 (gsi=8)
[    0.274816] xen: --> pirq=17 -> irq=12 (gsi=12)
[    0.274848] xen: --> pirq=18 -> irq=1 (gsi=1)
[    0.274881] xen: --> pirq=19 -> irq=6 (gsi=6)
[    0.274917] xen: --> pirq=20 -> irq=4 (gsi=4)
[    0.274957] xen: --> pirq=21 -> irq=7 (gsi=7)
[    0.284782] xen: --> pirq=22 -> irq=23 (gsi=23)
[    0.555651] xen: --> pirq=23 -> irq=28 (gsi=28)
[    0.555882] xen:grant_table: Grant tables using version 1 layout
[    0.619808] xenbus_probe_frontend: Device with no driver: device/vbd/768
[    0.619809] xenbus_probe_frontend: Device with no driver: device/vbd/5696
[    0.619810] xenbus_probe_frontend: Device with no driver: device/vif/0
[    0.635129] systemd[1]: Detected virtualization xen.
[    0.997567] xen_netfront: Initialising Xen virtual ethernet driver
[    0.997973] vbd vbd-5696: 19 xenbus_dev_probe on device/vbd/5696

Can you give me a please advice which type of virtualization am I running? Also how you convert HVM to PVHVM on CentOS 7? It should be common question but I found very little about it. 

Thank you,

Pavel

[Tobias Kreidl]

What version of XenServer is this?   The virtualization methods supported changed at some point as I recall. See also: https://sites.google.com/site/yetanotherocmoriginal/home/procedures/miscellaneous/convert-xen-hvm-to-pv

As to which VM virtualization method is running, I'm not sure if that's shown or not in "xe vm-list uuid=(UUID-of-VM) params=all" but see also

https://serverfault.com/questions/511923/determine-which-guest-is-running-on-xen-hvm-or-pv-guest

[Mark Syms]

CentOS 7 will run as an HVM guest. PVHVM is only used in special circumstances to wrap true PV virtual machines into an HVM shim. In most cases HVM is what you want.